In a lot of species, including primates, it is "normal", but males kill *unrelated* young. Since most of these species have a social organization where there is only one male who gets to mate with all the females, they are all his offspring, so they don't get killed. That is how the species survives. When he gets overthrown due to injury or death or what have you, the new male typically kills the offspring so they can immediately get pregnant again and invest in his offspring (being so invested in their offspring hurts them in this way). It's all off topic at this point, but I just find it interesting. However, female primates are generally very good mothers, and they do invest so much in one child. They watch their infants very carefully. Their babies cling to them from birth, though, so they can go about their business with the baby there 24/7. The fact that human babies lack that ability, which it seems all other primates have, is interesting.
Right, I'm not arguing that a parent who sees a child dying in the car, or anybody, should not rescue that child! I'm saying that decent people can forget that they didn't drop off a child at daycare, or not realize how hot a car is. I'm not excusing people who realize someone is baking to death and do nothing, relative or not, baby or not!
As for the charges, if the argument is he put him in there on purpose (child cruelty charge), then wouldn't that just be a lesser included offense of murder? You wouldn't need murder in commission of a felony - that's first degree murder. Felony murder is generally used where you are committing a felony not related to hurting the victim, and the victim dies as a part of that crime. I'm wondering if the charges indicate something else. If he was accidentally killed during some sort of abuse, and then placed in the car as a cover up, the cruelty could be in response to the abuse. Felony murder is often used where a third party did the actual killing as part of a felony the charged person participated in. However, as the victim is a young child, I can't think of what that felony would be. If he died as the result of abuse, that would just be murder - you wouldn't need the felony. You can't intentionally assault someone to the point of death and claim you didn't intend to kill them, even if you didn't actually intend for them to die. It still usually will count as murder.