The Lorax/Fridge

Fridge Brilliance

 * In the "How Bad Can I Be" sequence, at one point, The Once-ler's body begins to grow bigger than the land around him; in other words, he, like his corporation, starts biggering.
 * As does his ego.
 * It seems a little odd that the Once-ler's business growing and him becoming rich was condensed into a three minute montage (cool as it was) in the film, since that took up most of the bulk of the story in the book. But the Once-ler is the one telling the story, so of course he'd want to skim over the part where he turned into a corporate douche.

Fridge Horror

 * OK, so the Lorax sent the local fauna away to find a better place to live. Fine. But how many casualties were there before that happened, that we didn't get to see? I get that Dr. Seuss, generally speaking, is a Never Say Die kind of author, but at least a few of those Barbaloots had to have died at some point, whether by a tree falling on them, or starvation as the trees were chopped down. And, while we're on the subject, we see the humming fish climbing out of the polluted water. Unless they were actually amphibians (admittedly, not out of place, seeing as this is Dr. Seuss we're talking about here), they drowned in open air (since they were, you know, fish). Put another way, the Once-ler caused the extinction of the humming fish!
 * The Humming Fish were seen on land before the water got polluted, so my guess is that they are amphibious. Most of your point still stands, though.
 * The Once-Ler spend HIS ENTIRE LIFE regretting his actions, you can only wonder the kind of thoughts he had over the years....