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Is there a downside to slaughtering everyone I meet?

I was ambushed by a named enemy not long after jumping off the first tower and being unleashed into the vast world. I defeated him, and the game made a big deal about his death, awarding me Power and crossing off his name like I accomplished something useful.

Are there a finite number of enemies like this? Is there some downside to murdering him? Is there something beneficial I could have done with him had I spared his life somehow?