How do cops/detectives keep their cool?

I'm sure a lot of you have heard about "Baby Grace". The little girl whose body was stuffed in a storage bin that washed up on shore in Texas. I was reading the paper today and the little girl's 19 year old mother described to the police how her 2 year old died. This poor little girl was beaten with a belt, had her head held under water and THROWN across the room where she landed on the ceramic tile. If I was the cop/detective and she was telling me what happened I would want to throw her and her new husband across the room. I give law enforcement a lot of credit for keeping their composure and not hurting people like this. And WHY didn't the mom just leave the toddler with her dad? Why don't people like this just put their kids up for adoption if they don't want or love their child. I know we cannot understand it. Because to understand this behavior you have to think like them and I don't think like them.
I'm sorry to bring you down.