I'd say that invading someone's property and taking something of value from it is clearly wrong, if there are not significant mitigating circumstances. So it all boils down to are there sufficient mitigating circumstances. Hacking China's computers to save the life of a dissident by publishing info is clearly fine, even if illegal. Hacking a private citizen's personal email to expose their love letters or a company to expose their confidential business strategies is clearly wrong, even if legal (which I'm pretty sure it isn't).
You also have to factor in the foreseeable consequences of a leak. If it's going to result in innocent people getting killed (e.g. people who report on Taliban abuses, dissidents in tyrannical regimes) then it is no different to pulling the trigger yourself.
I haven't seen the info on wikileaks so I don't know to what extent Assange and his group fall into those categories. But given his apparent carte blanche approach to leaking info, IMO it's only a matter of time before he becomes morally culpable for numerous homicides, at which point it'd be morally justified for anyone in the world to kill him. Then again, many of the world's leading statesman - past and present - have been in similar positions, morally speaking.
His intent is probably noble, but intent doesn't really matter - you are just as dead if killed by a saint as by a sadist.