How long was it from the time he fired his last shot and when the police decided to kill him?
What difference does that make? He was still armed and threatening to kill more cops. They sent in the robot and gave him the choice to surrender or die. He chose to die. I’m fine with it. Why should the cops give him the chance to kill more when they had a safe alternative? Why does it matter if a cop pushed a button rather than pulling a trigger, other than it didn’t put them at risk?
Sounds to me like you sympathize with the perp more than the cops.