He described 44-year-old Keith as "the baby'' in the family of five boys.
"He does have a bad temper but not bad enough to turn around and say he's going to kill someone.''
Derek Kahi referred to some of the evidence in court about his younger brother, including that he was a user of P.
"He lived a different life than we did and he paid for it.''
McDonald was also in to P. In his evidence to the jury last week, he described himself as a "mid-level dealer''.
He told the court that he was not "primarily'' a drug dealer but instead made his money from cracking codes on stolen laptops and cellphones.
"It was known I could get things, whatever anyone wanted, and sometimes I was paid handsomely for it.''
He shared the home with his cat Tigger and Mr Kahi, who was moving out on the day of the shooting.
When police raided McDonald's three-bedroom home in east Auckland they found it had been rigged with security cameras and sensor beams guarding the driveway.
Inside, McDonald had stashed money, drugs and computer equipment in three different safes. He had also bought a high-powered air rifle to scare away robbers.
But McDonald used the air rifle to shoot his Mr Kahi.
During his cross-examination of McDonald, Crown prosecutor John Dixon asked if the argument was over a woman.
"He tells you, you're crazy if you think the two of them are together and that he's sick of your paranoia ... You get angry and you shoot him.''
McDonald denied it but Mr Dixon returned to the scenario in his summing up.
"He thinks his best mate is messing around with the girl he fancies. It's the oldest story in the book: jealousy.''
McDonald will be sentenced in December.
Warrick Kahi said he hoped McDonald would be "put away for a long time''.