That's not something anybody in this thread seems to want to consider a possibility, Nefrodite. They don't seem to think there's any chance that he feels bad about this, they seem to think he must have done it deliberately, willfully, and knowingly. From my own personal interpretation of the limited information provided, I think it's more likely he just did something stupid and wasn't thinking, however I'm not on the jury, and I don't have every piece of evidence.
Incidentally, rh71 asked and I don't know if you got an answer, according to the previous article in the other thread, the guy called 911 himself when he found the dog unresponsive. Naturally there is no other information given, such as what his emotional state was at the time (whether he was upset about the dog being hurt at all, or if he was just upset that the dog was dead and that he'd now have to get in trouble for it) and as you mentioned, no information about what the brother thought. He may, for all we know, consider it a total accident and not want any trial at all, or he may be blaming himself for leaving it with his brother knowing how much he hates dogs.
Nefrodite: The "blood boiling" concept is based on the fact that at lower pressures, liquids boil at lower temperature. Water boils at 100 degrees Celsius at one atmosphere (the unit of measuring atmospheric pressure), but at lower pressures such as on a mountain, it boils below that temperature (this is part of why food packages have "high altitude" directions). So some people are saying that the pressure inside the car could have been reduced so much, that the extreme heat in the car could have been enough to cause the blood to boil.