I think that the options of him shooting up his fellow soldiers was a concern in Lamo's mind, if you read around.
For me, a big part of the problem is that this is Adrian Lamo, and he has always seemed like an attention-seeking narcissist, even before any of this Bradley Manning stuff came up. Don't you remember when he was "the homeless hacker," and he spent his days sponging off of online acquaintances, trashing websites, and telling his story to any reporter who came along? I just have a hard time believing that his decision to turn in Manning was motivated by anything other than his pathological need for attention.
Another part of the problem is that whatever the ethics of what Manning did, it should be clear to any rational person that his treatment by the military has been totally egregious, way beyond the pale of any logical mode of incarceration. The man was not arrested for any violent offense. He's not a rapist or a murderer (whether you think his disclosure of information indirectly cost lives is irrelevant here). After so many years locked in solitary confinement, just what do they think he could be smuggling under his ball sack that they have to strip search him and force him to sleep on the floor naked every single day? Do gang lords have to do that? Did Jeffrey Dahmer?
It's hard to have any sympathy for the government's position when their opening move in the affair was to strip Manning of his basic human rights -- not just civil, but human -- and announce that they planned to keep him that way indefinitely. How can any American justify that, given the circumstances?
Sure -- if Manning did something wrong, put him on trial and lock him up. In a prison. Like a criminal. But what kind of society are we if our government can choose when and where it feels like following our most basic moral principles? I think it's this that pisses most people off about the situation -- not that Manning is some kind of hero, which is clearly questionable.