Kaidan sees the blue fire but doesn't lose focus on the bandit he's fighting. The reave is doing its job of hurting the bandit enough that it can't heal from its injuries, and causing it enough pain to drop the pulsefire weapon. Kaidan decides to take a risk and move it with his biotics, nudging it out of the bandit's reach and in Rosianthe's direction.

But he's hesitating in actually putting the creature down. However cruel and horrific their actions were, however furious he is at what they did to the town and its people, he can't quite bring himself to do the same. He debates on this, weighing out the scenarios in his mind. Kaidan approaches the bandit, still holding it with the reave, and slides out his omni-blade.

And stabs down at whatever works for a chest in the creature. Presumably still a killing blow, but a hell of a lot more merciful than the pulsefire.