You are unlikely to seriously injure someone with a single blow. It can happen, especially if you KO them and their head hits the ground hard, but it's not likely to. And your conscience should feel clear on this if it does happen, because they created the situation. Many of my techniques are designed to destroy (elbows, wrists, etc.). I won't hesitate to use them if someone puts me in danger and the technique is handy. Those are very likely to leave a long-term injury, but he was trying to injure me, so I'll do what I must to defend.