Santana, 47, went to Harris' house and confronted him over the feces, telling Harris, 27, to "clean it up," according to evidence submitted in the trial.
That's when Santana grabbed him and Harris pulled his gun out, Harris testified.
"He shot him seven times," Watson-Stokes said in closing arguments.
The incident occurred about 4 p.m. on the front porch of Harris' house on the 6500 block of Torresdale Avenue.
Under cross-examination by Watson-Stokes, Harris insisted that Santana was angry, confrontational, and smelled of marijuana.
Harris said Santana grabbed him in a bear hug and was lifting him off the ground, then tried to get the gun: "I managed to hold my weight and pull it away from him. Then I took two or three steps back."
Watson-Stokes repeatedly questioned Harris about why details of his struggle with Santana were not included in his statement to police.
"When I gave that statement, everybody was putting me under a lot of pressure," Harris testified. "I just killed somebody. I never killed anybody before and I was traumatized."
Watson-Stokes said Harris showed "no remorse" on the stand. That prompted Harris' lawyer, James Berardinelli, to motion for a mistrial. Judge Barbara McDermott denied the motion.
The jury also heard from Homicide Detective Hank Glenn. Harris' cousin Clifton Williams was inside Harris' home at the time of the killing and told Glenn he heard gunshots but did not hear Santana threaten Harris.
The jury is to begin deliberations Friday.
Contact Sulaiman Abdur-Rahman at firstname.lastname@example.org, or follow on Twitter @sabdurr. Staff writer Joseph A. Slobodzian contributed to this article.