This is probably right:

people who are hurting on the inside, then to hurt others on the outside

The worst thing is that this could create a negative dynamic in which people hurt another one, and then the second person does the same to a third one, and the third does the same again and so on...

Call me naive, but I like to believe that people are fundamentally good and noble,

I think the majority of people are like that, but there is a tiny minority who aren't good in my opinion.