Look at the incoherence in your post, yo start saying "What makes one life worth more than another? There is no argument that you could make that would convince me it's ok to kill something and not another" and you finish with "You are drawing a line somewhere between spiders and cats, I'm drawing that same line between cats and humans". You are contradicting yourself.
It might sound very beautiful to say every life worth the same, but can you compare kill an insect with kill a human being? because I'm not even going to enter on that debate.
I'm sure everybody here has ever killed a mosquito. Someone has killed a dog?
Would you think the same of someone who killed an spider because it was annoying that of someone who has killed a horse because it was annoying? Oh come on...
You kill cats, but you don't kill humans, so you don't think every life worth the same. But it seems you and me have the cats in a different appreciation scale.