In currently running season 3 of "The Vampire Diaries"..
When Damon attacked Klaus with Oak tree stack, Stefan came to stop Damon and saved Klaus.
The reason was given that hybrids were ordered to kill Damon if Klaus died. Why was this a matter? Stefan knew that it was the last chance to kill Klaus. When it comes to love for brother, hybrids weren't a big deal because they were in limited numbers who could be killed easily by Salvatore brothers. Bomb against hybrids were designed to weaken them too. Plus, the big point: Mikael was alive at that time to compel all hybrids (hybrids were part-vampires & Mikael already showed the compulsion on them). So, hybrids were never able to follow orders of Klaus. Then, why did Stefan screw things up?
In currently running season 3 of "The Vampire Diaries"..
I am going to break this answer into multiple parts.
Why was Stefan motivated to kill Mikael instead of Klaus?
It's explained at the end of "Homecoming". Klaus ordered his hybrids to kill Damon if Klaus was killed. Presumably some of Klaus's hybrids were not compelled by Mikael so Stefan believed the threat was real enough to save Klaus in order to save Damon.
Katherine: He doesn't know where all went wrong...
(She looks to the passenger seat; Stefan is sitting there)
Stefan: He doesn't need to know.
(Katherine stops the car)
Katherine: Are you good from here? I need to put about a million miles between myself and Klaus as soon as possible. Stefan?
(flashback to Salvatore house where Stefan's still down on the floor after Mikael's attack)
Katherine: Stefan. Stefan.
Katherine: Not exactly... (she gives him a bloodbag) Pep up! I've only got a minute before they realize that I'm not at the party...
(Flashback to the party where Klaus meets Elena/ Kathrine)
Klaus: You seem nervous.
Katherine: I'm not nervous, I just don't like you.
Klaus: People have been after me for a thousand years and I'm always one step ahead. So whatever it is you're thinking of trying, go for it! Give it your best shot! You won't succeed!
Katherine: It won't be for luck of trying.
Klaus: Ah, you'd be smart to tell Damon to mind his manners tonight, because if I die, I've already ensured that he'll die along with me! Even in death my hybrids have their orders. So, you kill me, you're killing him too!
(back in the car with Katherine and Stefan)
Stefan: How did you know that I would stop Damon?
Katherine: I didn't. I was just hoping that you would want to...
(flashback to Salvatore house, Stefan drinking blood, Katherine is with him)
Katherine: If Klaus gets killed he is taking Damon down with him.
Stefan: So pull the plug on the plan.
Katherine: And face the wrath of Mikael? We'll all be dead! Okay, look. I know that you've turned off your humanity and that you don't care, so there's only one solution.
Stefan: What's that?
Katherine: Care, Stefan!! Care enough to safe Damon's life. Because I am going back to that party and I'm seeing this plan through. Klaus will be killed. We'll have our freedom. But then Damon will be dead! Your brother will be dead, Stefan. Unless you care enough to do something about it.
This is further explained in "The New Deal":
Damon: That is for screwing up my plan! You stop me from killing Klaus and you steal his family! Why?? Doesn't make sense! Answer me!!
Stefan: Piece by piece Klaus took everything from me. I'm doing the same to him.
Damon: Go ahead, Stefan! Why did you screw it up?
Stefan: I did it to save you.
Damon: What?? No!!! No way!! You didn't do this for me.
Stefan: He was one step ahead of us. If Klaus died, his hybrids would have killed you.
Damon: I wanna yell it through your head! Stop saving me!!
Assuming the Salvatore brothers could kill all the hybrids easily, why did Stefan not go through with the plan?
Just because the Salvatore brothers could kill one hybrid, does not mean they could fend off a large number of them. Stefan did not know for sure how many hybrids were at the party so he would be risking that Damon could get bitten by a werewolf. Klaus's blood is the only way to save a vampire from a werewolf bite. If Stefan killed Klaus, then there would be no way to save Damon if that happened.
Assuming Mikael could compel the hybrids, why did Stefan not go through with the plan?
It has been established that Mikael is not trusted by Damon. You could assume from this that Stefan also does not trust Mikael. From the end of "Homecoming":
Damon: We thought of everything, Elena! Klaus having hybrids. Mikael turning on us. We brought in Katherine so you weren't in danger. Anything that could have gone wrong we were prepared!
Also, it has been established that Mikael does not have a problem risking the lives of or killing the Salvatore brothers in order to kill Klaus. One episode earlier in "Ordinary People":
(Mikael slams his fist into Damon's chest)
Mikael: Careful, Stefan. One move - and his heart's gone. Now, where is Klaus? What's the trick question, Stefan? One twist and he's dead. Unless you don't care of course.
Damon: His emotions have been shut off. He can't care.
Mikael: Really? His link to Klaus is so strong that he will let his own brother die? No ideas, hmm? Think, Stefan! I guess you're right. He's a dead end. And so are you, Damon.
Stefan: Wait. I can bring him back. I can lure Klaus back to Mystic Falls.
(Mikael takes his hand out of Damon's chest)
Mikael: And there is our loophole. Just had to rile him up a bit. Get Klaus back to Mystic Falls and I will gladly drive a stake through his heart. Fail, I'll drive it through yours.
Assuming the above two points, Stefan does not trust Mikael to care about saving Damon; Mikael only wants to kill Klaus. If you look closely at the conversation between Mikael and Klaus in "Homecoming", Mikael only says that he will compel hybrids that are attacking him. If hybrids were attacking Damon while Mikael was busy killing Klaus with the white oak dagger, then Mikael would not care about Damon's safety because Mikael is getting what he wants.
Klaus: Or I could watch my hybrids tear you limb from limb.
Mikael: They can't kill me.
Klaus: True. But it'll make a hell of a party game. All I have to do is rub these two fingers together and they'll pounce.
Mikael: The big bad wolf. You haven't changed. Still hiding by your playthings like a coward. You only forget. They maybe sired by you but they`re still part vampire. And they can be compelled by me.
(The female hybrid pushes Elena/Katherine to Mikael)
Stefan saved Klaus because Klaus had promised Stefan that he (Stefan) could have his freedom back for his loyalty. Additionally, Stefan knew that Klaus had compelled all his little hybrid minions to kill everyone if anything happened to him. So, Stefan was trying to save everyone by saving Klaus' life.
He saved Klause so he could get his freedom back and he could be released from his compulsion. It was mentioned by Klause in season 3 episode one that Stefan never forgets his family and maybe that was the reason why he saved Damon, it doesn't matter what Damon had done to him and how much trouble he'd gotten into, Stefan still loves his brother because he's the only family he has left. I don't think that Stefan would live with himself if Damon was killed. He still blames himself for Damon being turned into a vampire, I think saving him would be the least he could do!
1.it is simple, if an original dies, a link of magic broken among all of his bloodlin0e vampires in the World, so stef and damon will eventually die.
idk remember the episode but /\ it may not be revealed that yet, so second answer is below
- Mikael is already a vamp hunter, so
- Niklaus is affected Steff mind, if he die steff may lose his humanity foreva