This sentence has to be emphasised more, I believe:

We don't kill spiders either, unless they look dangerous.

In many situations, there is simply no need to kill. We can "just" explain them their place is outside/elsewhere (i.e. not in the house). We should respect all creatures and not kill any of them just for the pleasure of it. When there is no need, there is no need (and there is as well no room for a 'but').

Have a nice week-end!