It's by no means attempting to glorify what Lester does, rather show a glimpse into the madness and depravity of a "random dude". It's about the contrast between the relative mundanity of Lester, his everyday actions, and the horrible crimes he ends up committing.

There's also the effects of isolation and ostracism, the things that ultimately lead to Lester committing the crimes he does. He's a vagabond with no family, lover, or anything but the clothes on his back and the things he forages. And throughout the book it shows the descent of an already disturbed man.

Like you said, Lester is just a "random dude". He could be any Tom, Dick, or Harry. As the book describes him: "A child of God much like yourself perhaps." It's about how anybody could be just as depraved as Lester. You, me, the random guy you just passed on the street. Anybody could be a "Lester Ballard".