Most people kill someone if they could get away with it . Mysterious murders are a way to be educated about how to get away with it . However, as an audience, they also want to be sure that you really can not get away with it - which is why everyone roots for the detective
I think it basically boils down to fear. Fear is a great motivator . S. King wrote in his only non-fiction book ( Danse Macabre ) he believes that the reason for their horror stories are so popular is because modern humans have a constant need to remember that we are mortal - and these stories remind us that .
People like stepping over to the dark side as they identify themselves with it subconsciously, and love recoiling in horror that the possibility of what they are seeing exists, and are both simultaneously relieved it is not them and enjoy the vicarious pleasure of viewing someone else's pain and misfortune, as they themselves know misfortune and pain so that it gives them an outlet of a sort to become the voyeur instead of the sufferer.
Helping other people and deriving enjoyment from it is coming from a different part of the psyche, and is much more rewarding than the temporary wallow through the muck of depravity.
What's wrong with TV. I love TV. Drama is entertainment especially when people are killed. Shakespeare wrote many plays were people are killed. It is just entertainment. Worry about more important things. If you really want to help send money to the starving kids in Africa or something.