He deliberately strangled a girl during a sexual encounter and was later murdered by Zach Hamilton.
A lust driven murderer, Shawn strangled a girl during a sexual encounter. Afterward, he claimed that it happened accidentally and bragged about it, showing no remorse. Shawn had attended high school with Zach Hamilton who, unfortunately for Shawn, learned about the murder.
Zach was a budding serial killer who was tutored by Dexter Morgan. He learned from Dexter of The Code of Harry and what to look for in a potential victim. With his urges growing stronger, Zach followed the Code's guidelines and targeted someone from his past to kill that he felt deserved it - Shawn Decker. He spent a few days tracking Shawn to the Florida Keys, taking pictures of him, and setting up a motel room as a Kill Room, fully wrapped in plastic.
He approached Shawn and attacked him, intending to knock him out. However, Shawn didn't go down after the first blow, so Zach continued to pummel him until he died. Zach then shoved Shawn's body into the trunk of his car, where the Florida heat began to have its effect.
Meanwhile, Dexter Morgan arrived in the Keys and figured out where Zach was staying. Dexter intended to kill him (believing that Zach had strayed outside the Code and killed an innocent girl by the name of Cassie Jollenston. During his confrontation with Zach, Dexter learned that he didn't kill Cassie but instead had killed Shawn. After learning the details, Dexter scolded Zach about his careless murder of Shawn. Dexter then helped Zach get rid of Shawn's body while Hannah McKay, who had arrived with Dexter, dealt with clearing the kill room.