Legally speaking, I think there are two issues... Whether or not it was legal by Pakistan law, and whether the killing was legal but the geneva convention etc (whatever laws would protect someone from being killed outright). Now, whether the breaking of those laws was necessary or not is a separate thing.
Also, my opinion on this whole issue has me a little muddled up myself. I had a really good chat with an American friend as well as my Uncle who's lived in NY for 25 years, and they raised some interesting points, so i'm not totally sure on where I stand. There's both a humanitarian side to me, where the killing of an unarmed Osama doesn't sit right with me, no matter how evil he was, and the realist side of me that sees it had to be done one way or the other. Now i'm just not sure which I feel is more right. To be honest, I think it's too big a thing to look at in black and white either way. You've got to recognise both the merits and the moral implications of this incident, I think.
Either way, I don't think we're going to know the full story until 6 months to a year from now, but either way it will come out. Better to worry about it then rather than now when we don't have all the facts and emotions are still stirred up.