"Hey, the Devil is your friend? He will come here soon?"
Canon
At nineteen, Walter moved to a neighbouring town called Pleasant Hill and began to attend University there.
Despite living a 'normal' student life, Walter was still bitter, resentful and lonely.
While he was attending, three teenagers, Bobby Randolph, Sein Martin and Jasper Gein, approached him. They were fascinated by the Order and asked if he could show them the Devil.
Aged twenty-four, Walter began the ritual. He killed ten people in ten days, using a variety of methods. The constant between each victim was the removal of their hearts, and numbers carved into their flesh - 01121, 02121, 03121 and so on. The middle digit was actually a slash (01/21, 02/21, 03/21 etc.), counting them out of twenty-one.
Walter is arrested. When captured, he cried out that "He's trying to punish me. The monster... the red devil. Forgive me. I did it, but it wasn't me!"
Walter is tried and convicted for his crimes. Soon after he commited suicide, using a shiv he had made out of a spoon to stab himself in the neck and sever his carotid artery.
Headcanon
Between eighteen and nineteen Walter was homeless again, traveling between Silent Hill, Ashfield and other nearby towns.
As Walter grew older and lost hope in finding compassion from other people, Valtiel's influence began to overshadow his mind.
Walter discovered or was shown Joseph's aricle on the Wish House. This helped lessen what little hope he had left.
Walter fully intended to kill the majority of his victims; except for the animals int he pet shop, and the twin children he murdered. This is what he meant when he said "It wasn't me!" upon his capture.
Upon his suicide, Walter's soul and Valtiel fully melded together.
Adulthood & Ten Hearts Murders
Canon