Does Evil Prove Satan Exists?

I saw this article today and it made my stomach lurch thinking about what these people did to their sister: Brothers jailed over murder of toddler

It made me think that while I'm certain a benevolent God wouldn't allow an innocent to suffer like that, and therefore there must be no benevolent God, what does this say about a being that simply commits evil? Someone like Satan? Is it possible that the men charged with murder were possessed by demons? I know my parents would say so. My mom would be certain that those men were being controlled by Satan. It seems everyone around them was under some sort of shroud of evil, she would say, which is how the mother allowed it to happen to her own child.

How does one classify this form of abuse? I was about to say something about this not being your typical form of domestic violence but then what is typical? An abusive husband or father going too far and hitting their loved ones? Is that not evil? Is that normal? The sort of person who commits that type of abuse seems treatable to me. Not with an exorcism but with therapy and perhaps drugs (or maybe drugs was the problem?). At what point does an act of violence move from the realm of "normal" to that of evil, to be attributed to a supreme evil being?

Also, how does the abusive behavior go from being the act of a single, sick individual to having group participation? What force drives others to go along with something so obviously wrong? The easy answer, in my opinion, is to say that the devil made them do it. That Satan moved into their hearts. But to leave it at that opens us up to further acts of violence. If nobody studies the psychology of these types of events then we will continue to simply attribute them to evil.

What if, after lots of study, we are able to come up with a better list of warning signs to help prevent this type of abuse? In this case, there was obviously something going on as evidenced by this paragraph:

Social welfare groups admitted that while there had been concerns about baby Nia, she had somehow 'slipped through the net'.

Wouldn't it be nice if we could make a better net so children like Baby Nia wouldn't fall through? I think the only way to do that is to stop thinking small and attributing these events to evil. Humans are capable of great good and great evil and we must learn how to reduce the evil in the world.