Why do global warming deniers always feel the need to resort to lies, slander, and ad hominem attacks?

I think they do these things because they don't have facts or science to support their viewpoints. Another recent question on here by the so-called "top answerer" tried to link climate scientists with , even though the link it provided made no such claims at all. If you call them on their lies, they use their multiple accounts to get answers deleted. To be honest, I think many of them are so deluded that they can't distinguish between fiction and reality.