The guy pictured, Marshall Applewhite, wanted to fuck dudes and was ashamed of it so he got castrated. Then, after his best friend and cult's co-founder died, he convinced a bunch of people that the world was going to end and the only way they could survive was by committing suicide so that their souls could leave on an alien spaceship hidden behind Hale-Bopp comet as it passed by. Nobody can know what he was thinking but I'd wager he was just depressed and didn't want to die alone.

EDIT: Originally said it was Hailey's comet.