- Schulz's competitive nature: Schulz was a lot of things but one of the things he seems to not have been is all that gracious when confronted with competition. The idea of someone catching up to him or surpassing him in anything seems to have brought out the surly, ill-tempered, defiant child in him. The visual of a character standing around ranting in a blind rage because of a mild reverse in fortunes is too common not to have come from some drive within him.
- Sentimentality: It seems clear that he was also distressed by the idea of Farley ever passing on at all; it seemed to him that killing off the Pattersons' dog was a nasty trick to play on the readers. It so alarmed him that he was willing to become a four-panel Samson and more or less destroy his own strip to stop it.
- Love of timelessness: Another sticking point seems to have been Lynn's baffling-to-him decision to age the characters in real time; he might not have appreciated humor like this but he did agree with the idea of keeping the characters a specific age. In his mind, the best time scale was a sliding one; to him, the Pattersons should have been like every other family and stayed roughly the same age. The year on the calendar could change but Mike was supposed to be seven forever.
- The method: What really seems to have enraged him was the way it happened; in his mind, the poor animal died because a child didn't listen to her parents. What his constant hammering away at how if the stupid girl had not opened the gate, the dog would have lived tells me is that he had a bit of a blind spot. He seems, at least to me, to have been so focused on Elly and John saying that April should have obeyed them that he lost sight of what was to be obeyed. It would, I should think, simply never have occurred to him that since the 'command' was so vague and equivocal as to be worthless, his need to blame April made him look like a crabby, purblind old fool playing "blame the victim." He also seems to have forgotten that it's a parent's job to exercise diligence, to make sure that their children are listening to their advice instead of simply saying "Mission accomplished" and going back to drinking coffee and boasting about a cruise.
Then again, I could be reading too much into it; it might be that he was really irritated by being the victim of a vast game of bait-and-switch. Anyone of average intelligence would clearly see that when Elly looked out the window, said "So THAT'S how she does it" and told April not to ask a parent but a generalized someone if she could leave the yard, a catastrophe was inevitable; making crabbed-up, off-the-cuff remarks based on one's being deceived into thinking that Elly was competent would be a humiliation that he'd never own up to.