Ultimate forms in pic related are what told him to commit suicide
He tells Gwen that he has to die. Gwen begs him not to do so, but Ben tells her to keep quiet. He said that a true hero will sacrifice for the sake of others and that he must die, if he wants to set the Ultimate forms free to live their lives and then gives Gwen a goodbye kiss on her forehead
He walks to the pit of fire with the Ultimate forms around him, chanting the word "Burn". Before Ben jumps in, he stops to tell them that he never intended them to suffer. He takes one breath, waits, and jumps in.