The spider was a brown recluse, so it was a direct threat to me. And it was crawling around very near my bed, so it was in a direct threatening position- not because it would attack me, but because it would defend itself if I rolled over towards it in my sleep. The only endings for that spider would be to get it out of my dorm room or for it to die, and my previous attempts to herd it out of my room or peacefully coexist with it when it seemed content to stand on the other wall failed.
I tried to make it as quick as possible: not much, if any, give to the cardboard box I flattened it with; it wasn't even twitching. I usually filter my posts about psychic and magic phenomena, but I'll mention it here- I felt only the tiniest flash of pain and fear from it, for a split instant, because I didn't give it very long at all. Then I found its soul, and I explained myself, and I apologized, and I tried to help it recover from the shock of death, and it didn't even seem very upset about being abruptly disincarnate.
If it wasn't a spider that could hurt me, I wouldn't have hurt it. It was one of the dangerous ones, so my actions weren't unreasonable. So damn it, why do I feel so bad about this?