Some people I will never understand

I was playing ranked, and on my team was a zed support.

I was completely fine with that, If your good at something, play it, But in game, he said he is gonna troll. We asked him why, and his response was "I am trolling because i got a troll last game and I want revenge" So he went on to spend THE WHOLE game in the jg.

I don'd understand. Why waste 40 minutes of your life to waste 40 minutes of another persons life.