Yes, that temperature thing is real. I though that was known.
It's even used as an example of spurious causation in sociology and criminology: ice creams is sold more in the summer, violent crime goes up in the summer, therefore increasing ice cream sales causes increases in violent crime.
Now that's obviously not the case, but other suggestions are simply that people have more reason to be outdoors and thereby more likely to encounter a bad situation because more people are outside during summer. I could even to that that by law of averages corrected for the length of the Earth's days, the amount of incidents might actually be exactly the same once standardized.
If that's not the case (a significant remainder of cases not within the explanatory range), then it's time to add heat stress to the hypothesis. I'll explain: moral behavior is impulse-rejection based, so basically willpower. Willpower however, has been demonstrated to be a finite quantity where after a certain amount of expending it, the brain just can't give a fuck anymore and will be much, much, much more likely to act on any otherwise negatively framed impulse. Like buying fast-food after a long day, impulse buying stuff you don't need, doing excessive gambling, doing substance abuse, and of course: violent behavior. Of course, a person must still possess a predisposition towards that behavior (and not apathy / depression) to actually have a neural circuit that can play out its routine (circuit: "I'm helping!" ) to cause that behavior. Someone who doesn't have that circuit won't respond in that matter right away, or even after pushing for it.
Boop.