A Seattle man accused in the unprovoked slaying of another teen was sentenced Friday to 20 years in prison. Tristan Appleberry, 20, previously pleaded guilty to second-degree murder in the slaying of Aaron Sullivan, an 18-year-old Seattle youth killed in the July 22, 2009, outside Appleberry’s Leschi neighborhood home.

In what was described by prosecutors as an “unjustified execution” of an unarmed man, Appleberry shot Sullivan in the back of the head as he drove from Appleberry’s home. Sullivan died almost instantly from the wound.

Writing the court, Senior Deputy Prosecutor Jeff Baird noted that Appleberry’s attorneys had insisted that their client was guilty of nothing more than manslaughter, arguing that a verbal altercation earlier in the day led to the slaying.

Baird disagreed.

“The law does not give a discounted sentence to someone who killed a person who offended him,” Baird said in a sentencing memo submitted to the court before Friday’s hearing.

“The defendant,” the prosecutor continued, “who was intoxicated, escalated a rather unremarkable — admittedly hostile but completely non-violent — altercation between two groups of young men into a firearm fatality.”