I think a large part of the POINT was that even in extreme cases such as this, a black & white OMG EVIL!! worldview is terribly inaccurate.
Kevin's worldview, as a superbeing, is essentially the whole buddist monk (or whatever) viewpoint: Cause no harm to anything, no matter how small & evil. Ie, don't step on the bugs. Even the nasty nasty bugs. But that cost him dearly and he essentially got really pissed, something snapped...and he stomped on the nasty bugs.
When one gets really pissed one doesn't tend to stop and say...Hmmm, these evil, horrible little creatures just killed my wife and destroyed my life...what are my options here? I think I'll make a list of alternatives... No. The whole point is that it wasn't a rational choice. Kevin made a mistake and it caused him to do something 'evil', but that doesn't make him 'Pure Evil'. That's Pure Nonsense.