Why did Newt Scamander carry a suitcase full of beasts with him when he knew that they are prone to escaping? Also he lied at immigration saying that he brought nothing and he put New York City in danger. I cannot imagine how irresponsible this character is.
Because the case of creatures was safest in the care of Newt, a trained magizoologist.
In figuring out why Newt made the decision he did, there are a few things that are very important to remember.
- There was no one more capable than Newt at handling magical creatures, and caring for the creatures in the case.
Newt's case contains several magical creatures, many of which are dangerous to the average wizard. From what we know of the wizarding world's attitudes toward magical creatures at the time, if Tina's statement is similar to what an average wizard thinks, they would mostly consider these creatures things to be killed or avoided, not cared for. Even if other wizards are tolerant of magical creatures, Newt is likely the only wizard at the time who is actually capable of caring for this variety of creatures. There was likely no one who would be capable of taking care of the case while Newt went to take Frank home. So, he took the case along with him so he could continue to take care of the creatures himself.
It's perfectly logical to think the best way of handling a case of magical creatures is for it to stay with the magizoologist who's trained and capable of caring for them, and despite the problems it caused, this was still probably the best solution. Remember, the case of creatures would have needed to be taken care of for as long as it took for Newt to take a steamboat to New York, return Frank to the desert in Arizona, and travel back. In that time, the creatures within would likely need some kind of basic care unless the case was enchanted to feed them using magical methods. However, we see Newt feeding some of his creatures, and one of Gamp's Laws states food cannot be conjured from nothing. Therefore, while this is not definitive proof because Newt could possibly just like feeding his creatures even if the case could do it automatically, the case may not have any way of feeding them itself. Anyway if Newt knew he wasn't intending to let the case out of his care, he'd only need such a function if he was somehow made incapable of caring for his creatures by sickness or injury.
Newt also has a special skill with his creatures. They seemed to mostly have some kind of affection or at least tolerance for him, getting them to cooperate with him and be more compliant. For example, Pickett the Bowtruckle is highly unlikely to willingly accept being out of Newt's care. They likely would be even more difficult to manage for another wizard of equal skill with creatures simply because they are used to Newt as their caretaker. In addition, being a skilled wizard in general doesn't mean someone can manage the creatures. Skill with handling magical creatures is different than skill at spellcasting and. A wizard who's gifted in Transfiguration might be terrible at brewing potions, so it's similarly wrong to presume that a good enough wizard can take care of the creatures.
- The only creature who escaped while the case was actually with Newt was the Niffler.
Most of the creatures escaped when Newt no longer had the suitcase, and it was instead in the possession of Jacob, a Muggle who had no idea of the contents. When Newt had the suitcase he was able to watch it better, and only the mischievous Niffler kept escaping. Newt also misplaced the Occamy egg, which was possibly in his pocket, but the hatched Occamy didn't escape while the case was with Newt. If the case was with someone who wasn't as experienced as Newt was in handling magical creatures (whether it was in New York or left with someone somewhere else) it's possible more creatures would have escaped.
- Newt's reason for going to New York was to return a magical creature home.
He needed a way to transport Frank from wherever he left from to New York, and keeping him in the case with the other creatures was a good way to safely transport him. It kept the other creatures within Newt's supervision, and also was a successful method of hiding a Thunderbird to get it past Muggle customs until Newt could return him to his native home.
- Newt couldn't be expected to plan for the majority of the events that took place.
The only thing Newt could have possibly been reasonably expected to have a plan for is the latch breaking on his suitcase, and maybe a way to secure the Occamy egg better. Depending on when it broke, Newt likely didn't have much time once he realized the Niffler could escape to do much about the latch on his case though. However, the majority of events that made the situation so chaotic were things that Newt could never have anticipated. These events made the situation with the creatures far more serious than it would have been otherwise, but they weren't things Newt reasonably could have anticipated happening.
Newt couldn't have expected a Muggle would be carrying a briefcase that looked exactly like his, and he certainly couldn't have expected that he'd find himself in the situation with MACUSA and the Obscurus wreaking havoc on New York. The Obscurus seemed to have started causing chaos shortly before Newt arrived, so it's unlikely he even knew something unusual was going on there. He was traveling by steamboat (presumably a non magical one) and would likely not have had any way of knowing the events in New York at the time. He also would have stayed much shorter there if he didn't have to retrieve his case, remember he was only trying to return Frank to Arizona.
In conclusion, while creatures did escape, it's likely that Newt taking the case of creatures along with him to return Frank home was the best plan. There was no one more capable of caring for the creatures within than Newt. He probably should have secured the case better, but why he didn't is a separate question. One which I actually asked a while ago myself: Why didn't Newt Scamander fix the lock on his suitcase?
Because he's not terribly self-aware.
Scamander falls into the "absent-minded professor" role. He knows a great deal about magical creatures ("book smarts"), but lacks basic awareness and common sense ("street smarts"). This is a standard character archetype which you will see in many different works of fiction. The story is largely about Scamander growing and becoming a more rounded character. The quest to recapture all of the creatures is a vehicle for this character growth:
- During the bank scene, it becomes painfully obvious that he had no plan whatsoever for any of his creatures escaping. He simply did not expect this to happen at all, and handles the situation very poorly (disillusioning himself, for example, would have gone a long way).
- In the scene with the Erumpent, he displays a greatly improved ability to improvise, but is still largely relying on book rather than street smarts (he's focusing on the Erumpent mating dance, not on the fact that Jacob is about to get impaled and/or blown up). We see the same thing during the department store scene (his primary function is as an encyclopedia, while the other characters do the "work").
- The escape from MACUSA marks a shift: Scamander's situational awareness is greatly improved and he is able to act adroitly and independently.
By the end of the movie, he is able to identify an undercover Grindelwald largely on intuition and an offhand remark by the latter.
At the beginning of the movie, Scamander simply does not have the common sense to realize that his plan is fragile and unlikely to succeed. His creatures come with him across the Atlantic, not because he planned it that way, but because he failed to make alternative plans for them.
It could be argued that there were no reasonable alternative plans to be made, but I must disagree with that. For example, Scamander could have easily left the case with Dumbledore, along with some simple care instructions (though he would then need some alternative means of transporting Frank to Arizona and/or wherever he was really going). If he did not feel comfortable with Dumbledore caring for them, he could have purchased a more secure carrying case, perhaps one which would only open to his touch, or with some other security mechanism (e.g. "Doesn't pop open by itself"). If even that was too much, he could have acquired a cheap padlock or (maybe) cast Colloportus on the case, or even wrapped a bit of twine around it as he does at the end of the film. He did none of these things.
This can perhaps be addressed by a deleted scene in Crimes of Grindelwald. When Newt and Jacob prepare to leave for Paris, Newt packs some baby nifflers in the suitcase and says:
I don't think it's reasonable to ask Bunty to cope with everyone in here and a bunch of newborns.
Jacob immediately points out that this is a bad idea, and reminds Newt what happened the last time he went traveling with his suitcase full of beasts. Newt responds that he's fixed the suitcase so that nothing can escape.
It is possible that the first time around Newt had the same thought process, and it also seems that he is somewhat naive about the ability of his creatures to escape the suitcase.