@IL: Of course, "I just feel that hurting living things is bad" sums the inner perspective quite well, but this isn't really an answer to the question why exactly hurting living things feels bad for me, and why I wouldn't take the pill that shuts down my mirror neurons.

By taking the pill, I create a people-hurter, a thing-that-hurts-people, which is undoubtedly a bad thing to do judging from the before-the-pill POV. It's not that different from pressing a button that says "pressing this button will result in a random person being hurt or killed every day for 40 years since this moment".