Kaidan has nothing to protect himself with and he refuses to use his biotics enough to drain him completely. Toning them down though... it decreases their effectiveness, their range. He won't kill Ken though. He refuses to. Kaidan's barrier crackles as the machete catches just the edge of Kaidan's forearm as he ducks back, skipping down and leading them into the forest, cramped quarters. He doesn't put on an offensive attack again though; just focuses on his defence. If no other option is provided, then... perhaps that would change, but for now, no.