On Teen Wolf Season 5 Episode 5, Donovan's death is presented as a moral quandary. In reality, it is black and white. When a chimera threatens to eat your legs, it's okay to stop them.
Donovan versus Stiles is predator versus prey. Stiles kills Donovan in self defense. Furthermore, Stiles releases the scaffolding to stop Donovan, not to kill him.
So, we can add "accidental" on top of "self defense." Let's not forget, Donovan was hardly dealing with a full deck before the Dread Doctors made him a science project. Stiles gets a pass in my book.