Christians: I had an Atheist friend in high school who treated me really mean and condescending..............?

I also had a friend for about a year, who was irreligious, and we hung out a lot and were close, but one day he suddenly ended all contact with me and started ignoring me, and refused to answer my calls or texts, which really made me upset. Anyway, sometimes I think of some of the people in my past who have wronged me, or made me mad, or treated me unkind or unjustly, and then I picture them all in hell, and in a way it makes me feel happy that these people are going to suffer one day. Is this really bad of me? Is it a sin?