but maybe it's not so much about what someone has to gain but what they have to lose.
Let's say a doctor wants to go out and leave his children and be sure they will stay asleep, so he gives them a sedative. A sedative which only a doctor would have access to, because let's face it, you can't just go out and buy these things over the counter. So said doctor would have to use his status to aquire drugs that, essentially, he shouldn't even have.
But he gets it wrong. He overdoses the child and she dies. Or she chokes in her sleep, but because she's sedated, she dies. If the doctor reports the death there will be an autopsi, and it will come to light that the child died while sedated.
He is a renouned cardiologist, an expert in his field. But he had used his position to obtain drugs to sedate his children so he could go out and have a bad time. He would almost certainly lose his licence, and therefore his career. And he'd killed one child, so he would almost certainly lose the other two, as it would then become apparent that he had sedated them too.
Far more to lose than just one child.