Why did they try to kill Padmé with centipedes, when the droid could have just shot her once it had cut the window?
Is there any explanation beyond shoddy writing?
Jango thinks they're more subtle.
If Jango was having any similar thoughts about her, his helmet hid them. “We’ll have to try something more subtle this time, Zam,” he said without preamble as soon as she was close enough. “My client is getting impatient.”
I’ll bet, Zam thought, but she only nodded.
“There can be no mistakes this time,” the other bounty hunter went on. “Take > these. Be careful. They’re very poisonous.”
The Databank indicates that their venom is especially deadly to humans
Their venom is particularly deadly to humanoid species, which is why Jango Fett gave a pair of them to Zam Wessel in their attempt to assassinate Senator Padme Amidala.
They're untraceable and can evade security
"Zam uses two deadly kouhuns for her Senatorial assassination Job. Small, silent arthropods like the kouhuns of Indoumodo can evade even tight security. Unlike projectiles or energy weapons, they are virtually impossible to trace back to their users. Kouhuns are starved in advance, so they head straight for warm-blooded life forms when released, and use a fast-acting nerve toxin to kill their prey."
They're very reliable, very deadly and able to evade the most sophisticated surveillance equipment
Kouhuns actuolly have two separate toxin-delivery systems. The stinger on the end of their foil can deliver an excruciating venom that leaves the victim writhing in agony. Although this is not fatal, it allows the kouhun to employ its mandibles. These release a fast-acting toxin that can kill large animals in seconds. For those who have already been stung by the tail, the end comes as a welcome reprieve from the pain.
Another reason kouhuns are favoured by assassins is that they are expert stealth hunters. They are able to bypass the most sophisticated / security systems with an almost Force-sensitive awareness. The pair of kouhuns that almost killed Senator Amidala, for example, managed to scurry just a short distance in front of an astromech droid programmed to be on alert. If it had not been for the two Jedi standing guard outside the room, the kouhuns would surely have feasted on Naboo flesh that night.
Luckily for the Senator, kouhuns instinctively strike at the neck, even though a jab to a toe would be just as fatal. The time the kouhuns took to scurry up Amidala's body to her neck gave the guardian Jedi just long enough to kill the creatures. Even so, Amidala knew that she had stared death in the face.
It's noted that their behavious is highly predictable and, because of their sheer rarity, largely untraceable.
A small group of bounty hunters, including Jango Fett, were aware of a creature, native to the planet Indoumodo, that could be used as the perfect weapon. Kouhuns are sizeable carnivorous arthropods that use their lethal nerve toxin to fell larger animals and consume their flesh. When hungry, kouhuns become entirely one-track-minded and will head for the first organic form. This makes them highly useful for assassins, as they behave almost as predictably as a droid - but without the disadvantage of being traceable.
Assassins are always looking for new ways to dispose of their targets. The best methods allow for long-distance application and cannot be traced back to the user. With modern forensic science, it is possible to trace bullets, bomb debris and even blaster scorch marks back to the owners of the weapons. Other weapons, such as vibroblades and more primitive devices, require the assassin to get very close to the target.
You may wish to note that this supposed 'security through obscurity' is the same reason that Jango used a Kamino Saberdart instead of a more conventional weapon.