When my sister was pregnant with her oldest, she made a statement one day that she was scared she would set the baby down somewhere and forget where she put it. Everybody sort of laughed it off, but she was serious and I knew she was really worried about something like this. She has crazy A.D.D. and I could almost see something like this happening. Thankfully, her youngest is now 2 1/2 and all is well, but I can see how it could happen. Someone like me, who is not so mentally scattered, no. But here's the thing...she loves her babies every bit as much as I love mine and she is a great mom. So I try not to judge the parents too harshly if their child's death (hot car or otherwise) is a true accident. Like someone else said, go ahead and toss them in jail if you want. It won't compare to the mental torture that they will suffer the rest of their lives.
People who do it on purpose, I say get CPS involved immediately and yep, arrest the parent on the spot.