Why do people even do this

I was in a match with this guy, and he flamed me because of how I couldn't win the game for them. Next champion select, I got the same guy again. And he did something pretty terrible. He told everyone that I was flaming the entire match, that I said that I have no support, and that I was trolling. But now the thing is that I didn't do ANYTHING from what he said. In fact, I was pretty positive (except that I said that vayne is too strong), and I was trying to help my team. So, why do people lie and make stuff up to make everyone worried? This is something new to me.
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