Please keep in mind that what I am about to say does not reflect my current beliefs.

Unless the situation is dire, where I am from, humans actively attack and kill all non-human life forms. It is part of their religion to systematically commit genocide. To get the humans I was combating with to agree to a peace treaty with our force, I had to destroy half of the battlecruiser and then take the Admiral hostage. Even then, he preferred that I kill him as opposed to even thinking about a cease fire.

If it were not the interference of the Space Marine captain and an outside threat, I would have had to kill him.