The man charged with shooting Tommy Speed to death in a hallway of the Lenox Street Housing Development two years ago had a motive and a plan to kill the rival drug dealer, a Suffolk County prosecutor told Superior Court jurors this morning in his closing argument.

Assistant District Attorney Edmond Zabin told the court that GARRETT “G-WHIZ” JACKSON (D.O.B. 7/13/87) had “a purpose and a plan” when he tracked Speed down, put a gun to the back of his head, and “executed his competitor.”

Jackson is charged with first-degree murder for allegedly shooting Speed on the night of Feb. 11, 2009, after deciding that “he had had enough of Tommy Speed,” Zabin said.

Zabin said Speed sold crack cocaine on the street level and at wholesale prices to Jackson and his associates. But the product that Speed sold to Jackson was inferior, which resulted in Jackson losing clients, Zabin said.

“Tommy Speed was the guy everyone went to because he had the best product,” the prosecutor said.

“Tommy Speed was making too much money in Lenox Street,” Zabin told the court. “So [Jackson] put a gun to the back of his head, sending a bullet crashing into his skull.”

Because the defendant and the victim knew each other, Zabin said, Jackson was able to get close enough to Speed to “do what he told everyone he was going to do.”

“It’s the very picture of deliberate premeditation and first-degree murder, and that is what happened in that hallway,” Zabin said.

What Jackson had not counted on, Zabin said, was that a female customer who observed the shooting would go against “street logic” and share what she knew with police.

“The defendant had every confidence that he was home free,” Zabin said, “because the only person between him and Tommy Speed was a crackhead.”

That witness, and the testimony she gave to the court, “was devastating to the defendant,” the prosecutor said. “She identifies Garrett Jackson as the shooter. Think about what she said and why she said it,” Zabin told the jury. “To [her] and so many others, Tommy Speed meant something. He was a friend. Somebody they respected and who respected them.”

Even though the eyewitness’ testimony “in and of itself is enough to convict Garrett Jackson,” there was also “evidence of a motive from a variety of sources,” Zabin said, including statements the defendant made both before and after the murder.

Zabin also acknowledged the “meticulous” investigation by Boston Police homicide detectives, who returned to the scene every day and interviewed more than 80 potential witnesses, documenting every step they took, in the aftermath of Speed’s death.

Motioning to the defendant, Zabin told jurors, “On the night of Feb. 11, 2009, this man stuck a gun at the back of Tommy Speed’s head and pulled the trigger …. The time for him to be accountable for what he did is now.”

Catherine Rodriguez is the DA’s assigned victim-witness advocate. Jackson is represented by attorney Barry Wilson. Jurors will begin their deliberations after Judge Patrick Brady instructs them on the law.