Const. Craig Ashman was one of the members of the Emergency Task Force who was trying to arrest and subdue Kachkar on the morning of Jan. 12, 2011.
Kachkar had struck a garbage truck and the Emergency Task Force had him surrounded.
Officers had first tried to use Tasers to stop him.
But when the effect of the stun gun wore off, the plow lurched forward and Const. Errol Watson found himself trapped between the snowplow and the garbage truck.
With Watson screaming that he was stuck, Ashman made the decision to use his gun.
"If the plow keeps going forward he's going to die," Ashman told jurors on Wednesday.
Another ETF member, Const. Andrew Bozzer, testified that he was having the same thought.
"I need to get to my gun, because one of us is going to get killed," Bozzer testified.
Ashman shot Kachkar twice.
Watson was pulled to safety and Kachkar, who was not wearing shoes or socks, was placed under arrest.
Firefigher Eric MacMillan treated Kachkar at the scene.
MacMillan testified that Kachkar was talking about his mother and the Russian government.
"You don't understand, it's all a Russian video game, it’s all on Facebook," Kachkar told him.
About an hour before Kachkar was arrested, Russell had been fatally injured when he was struck by the snowplow. The 35-year-old officer left behind a wife and young son.
At the outset of the trial, Ontario Superior Court Justice Ian MacDonnell told jurors that there is no dispute that Kachkar was driving the snowplow that day.
What is in question is Kachkar's state of mind at the time.
Kachkar has pleaded not guilty to charges of first-degree murder and dangerous driving in Russell's death.
Wednesday marked the ninth day of Kachkar's trial, which began earlier this month. The trial had been delayed for a day when a juror fell ill.
Jurors have previously heard testimony about Kachkar's background, the fact that he had been looking for work and that he had stayed in a shelter the night before Russell died.
The trial continues on Thursday.