Self censorship

When does it become acceptable to write something that's funny at someone else's expense? By which I mean, when is it OK to ridicule an actual, real person? You know, those things that have actual feelings and lives that may be affected by public ridicule.
No matter how much we don't like someone, no matter how much we think they "deserve it", there is ultimately a person on the receiving end. Even if we don't think they're much of a person. So what's acceptable when we're dealing with another human?
Looking around, it seems attitudes range from "anything goes" (no matter how cruel, damaging or untrue it may be) to "if you can't say anything nice, don't say anything at all." I tend to be somewhere in the middle but get tipped more towards "anything goes" the more of a public figure someone is. I see it as a case of live by the sword... the more you make your living from being in the public eye the more you have to deal with the fact that the public eye can sometimes be cruel and unfair.
For me, that generally means politicians, movie stars and TV personalities are all fair game. In terms of ordinary people who find themselves in the public eye (usually for doing something stupid) I try not to be too cruel. And the one thing I always try to avoid is the internet lynch mob mentality. The people who indulge in internet vigilantism have a tendency to justify their action by saying their target has done something really bad. But I wonder how many of them would escape unscathed if their own life was subject to the same scrutiny.
The reason I'm musing on this topic is a news story that seems like perfect fodder for me. A pair of New Zealanders described as "experienced pilots" have put themselves out front for this year's Darwin Awards with their method of demise over the weekend. They were killed when they crashed a microlight plane they took for a joyride. In the middle of the night. While it was raining. And foggy. And there was lightning. To top it off, police believe the two had been drinking at a party before the flight (which could explain why they thought a midnight joyflight in shitty weather was a good idea).
Now, these guys seem to me to pretty much fit the definition of "asking for it". But at the same time, I'm sure there are people who are very sad they're dead and wouldn't appreciate the loss of their loved ones being made light of. I think I'll hold off saying anything too disparaging until there are more details confirmed (like whether or not they had definitely been drinking) but I suspect the nature of this story means these guys are going to be slammed around the world.
Of course at the back of my mind in all this is I'd really rather not have all my personal details used for online fodder. Quite a few people have a go at the fictional entity of Mr Angry already (many of whom seem to have no clue that this persona isn't actually me) so I'm always kind of expecting a wider public "outing". I'm not stupid enough to think that if I'm nice everyone will be nice to me but at the end of the day I'd rather be able to hold my head up regarding my own actions rather than worry too much about what someone might say about me.