> Why is okay to kill a tree to build a home, kill a plant to eat, but not okay to kill an animal to eat? > Does the biological complexity of the organism make it more or less okay to kill it?

The ability to suffer is the distinguishing/relevant factor. We all know what suffering feels like and we know that animals have the capacity to suffer. We don't really know that trees do. I want to reduce the suffering I am responsible for.

> Is it okay to kill a cockroach or a rat in your home?

I probably would get rid of infestations in my home and feel bad about it.

It's not about having a 100% perfect record with not killing animals. It's about striving to minimize animal suffering as much as practicable. You're never going to reduce this to 0 animals. But you can get to 95% better than the average human pretty easily if you want.