But the mutilation isn't malicious or done with any malintention. Irmgard Baty is clearly just curious about whether or not the spider will continue to walk and due to her innocence and overall curiosity (and possibly inability to experience empathy), she simply cuts the spider's legs off while being completely oblivious to its suffering. The experience heavily parallels mankind's experience with nuclear weapons – man foolishly developed massively-powerful weapons partially out of scientific curiosity, leading to the highly unfortunate (understatement of the century) situation that occurred on Earth prior to the events of the novel.
Ironically, Isadore, the "idiot" of the novel is the only one to point out exactly what will result from Irmgard's mutilation of the spider, but due to his perceived idiocy and the general obliviousness of the three androids he lives with, he is unlistened to. I wonder what kind of commentary Dick was trying to imply. Does society label those who know what's right as "idiots?" Or do idiots sometimes provide some kind of insight that a more analytic or rigorous mind cannot bring?