It is tempting to use harm to self as an explanation for the wrongness of various things. Kant infamously did this in trying to explain why it's wrong to be cruel to animals, namely because it dehumanizes us. And two commenters did so when I argued from the wrongness of attempted murder, in a case where the intended victim doesn't exist, to the existence of a necessary being.
Now, I agree with Socrates that every wrong action harms the agent. And I even think that sometimes the harm to self is the primary reason why an action is wrong--for instance, harm to self is the primary reason why it's wrong to use heroin. But in both the attempted murder case and Kant's case, the invocation of self-harm fails. Let's see why.
Normally when I do wrong, two main harms result to me:
- The action constitutes me as a wrongdoer, makes me be guilty.
- I develop morally bad habits.
Let's now think about the two main harms. To say that an action is wrong because it constitutes me as a wrongdoer, makes me be guilty, at best simply shifts the burden of explanation into the equally difficult question of why this action constitutes me as a wrongdoer (why does kicking a dog make one a wrongdoer while feeding it does not?). But actually it's even worse than that: it simply gets things the wrong way around, since an action constitutes me as a wrongdoer because it is wrong. So (1) won't be the explanation of the wrongness of the action. Though of course it is true that wrong actions constitute me as a wrongdoer, and I guess that multiplies the amount of wrong in any wrong action.
On the other hand, the second harm, that of developing morally bad habits, is a merely contingent matter. It would be wrong to be cruel to an animal or attempt murder even in the last moment of one's existence, when no bad habit were developed. Further, cruelty and attempted murder are wrong even if one's character is already so calloused that the action does not make it any worse. We can even imagine outlandish cases where cruelty and attempted murder end up improving one's character, say because a renowned neurosurgeon credibly promises to eradicate all one's tendencies to cruelty as soon as one kicks her neighbor's cat.