All right, I've been looking around the Spinpasta wiki and read a few Mario pastas. And what I saw in most of the stories (Like The Plumber that took me to Hell) is that Mario is normally a nice guy, but he's a psycho; thanks to Bowser kidnapping Princess Peach Toadstool over and over and over again. I think there might be another reason, but it's not really a theory, just a story I created out of my speculation. This is also ignoring the events of Mushroom Kingdom Massacre.
Mario was walking down Toad Town, he was very happy. How could he not be? It was good weather, Bowser had been gone for a very long time and Peach made the most delicious cake Mario ever ate in his whole life! Luigi had rescued him from a painting again while having to go through ghosts and the trolling of Elvin Gadd. And Luigi still didn't give up! Yoshi always was ready for having to carry Mario, but not a single complaint came from the dinosaur. Because Yoshi never let anything happen to the hero, and carrying Mario was part of that "quest". Yep, his family and friends sure gave him all the love he needs, and he was happy to give it back.
He came across the jungle. He was planning on visiting Donkey Kong, since he finally made true friends with him. But he came across a Goomba. The Goomba had red eyes and foam came out of his mouth. He saw Mario and stormed towards him, biting him with its sharp fangs. Mario then felt a lust for blood...
And that's how I think Mario became a psycho! Let me know what you think!