Hmm...the first tell-tale sign is the end effect...if it's something that's truly injuring yourself, it's bad. In an extreme, it is the action that matters, yes. But not all play is like that; much is very controlled and light.

Having done such things for both reasons, I can define a big difference: cutting and pain as an escape, as something to overwhelm bad feelings or emotions, is bad; it's good when it intensifies pleasure. That is, if you're feeling down when you start, then it's probably a bad way to deal with emotions; if you're feeling happy and content beforehand, then it can be viewed a lot more healthily.