Here's why
He was an evil, cursed being. He was also a super genius. He discovered the true meaning of life and the universe, but it wasn't pretty. He knew about civilization and how fire was a requirement. Since he was cursed with such large intellegence, he knew everything, even the grim truth that even we don't know yet. He was mad at life, at the universe. Instead of offing himself, he decided to take revenge by destroying many more lives. He walked onto land knowing the intelligence it would bring, and with intellegence comes the loss of nature and degeneracy. He laughs in his grave when he sees oceans and forests polluted, seeing all the species going extinct every day, seeing the slow degeneration of society. His true last laugh will be when we too, discover the truth that he knew all along. That would lead to the most despair, most devastation and destruction ever. His descendants would be played like pawns, playing into the wishes of the one who caused them so much suffering