@Shinhyung,
Mostly in response to the OP:
Consider the following: For some reason, I got a little paint on my wall. Not all too much, but it is clearly visible, and makes a weird shape. If I disliked it, I could paint over it or get it off, destroying it -- or I could leave it be, ignore it while I don't mind it. Whatever choice I made, I would not care what this paint thought of it.
The same about the spider: I could just leave it be, ignore it -- or, I could decide to rid myself of it. It would be a matter of mood, of fancy -- the spider is a noticeable irregularity in the otherwise insect-less space, just as the paint is an irregularity on the otherwise single-colour wall.
What makes a human different? For one, some humans are to me quite more than just a bother. I may enjoy their company, or have use for them. Thus, I would likely never want to simply squish these ones. As for the others, I lack the possibility to squish them.