They had several minutes to discern what was going on, they shot a man they thought had a gun.
They DID NOT shoot a man they thought had a hose nozzle.
Their intent was likely not malicious, they DID commit murder. The question is not whether or not they murdered someone, it is whether or not they were justified in their actions.
You are right it may not have been malicious, however it was certainly Incompetent and Negligent.
You can sit back and say you would have known it was a garden hose nozzle; however, I highly doubt that you would have.
I have been, and so have several of my Brothers in Arms been confronted by similar situations countless times. I have never Murdered a Drunk or Child for playing around.
LOL...This is a very circumstantial problem, it would be dificult to prove you would have known otherwise.