No danger? With the exception of the Hulk, direct fire from Chitauri weapons is likely to result in moderate to heavy damage. Thor being a weird gray area because while he should be tough enough to take it, it's just unclear at this point.

The Iron Legion isn't numerous enough.

Night night guns are an invention of AoS, and while AoS is definitely part of the MCU as far as they are concerned, the reverse isn't necessarily true. See how Coulson is not going to be in any of the further movies, last I heard.

Murder is legal terminology. It isn't against the law to kill someone (in legal terminology) if that was .. pulling the plug, self defense, soldiering, etc.

So if it isn't a question of semantics over murder, but of whether it was needlessly harsh.. No.

Why? Because Hydra is evil as fuck, does evil shit, and experiments on people, kills innocent civilians (see how the Avengers didn't when they attacked the compound, they blew up a bunch of armed extreme assholes,) and basically does whatever it needs to do, to become a world dominating force.. further, they don't surrender. Inhumanly so. They NEVER surrender. They swallow suicide pills to avoid being questioned. So even if you took them gently, they'd still kill themselves before being questioned.