It's highly unlikely that Data can make a decision without weighing all of the options as well as the likely impact it'll have on himself. That microsecond it would have taken him to weigh his options and to pull the trigger Data KNEW that if killed Fajo he'd be free and could contact Starfleet/the Enterprise and get back home and that'd he have to suffer the consequences upon himself for his actions.
What Data did was pure self-defense/defense of others. If Riker can beam down and vaporize a woman trying to kill the leader of breakaway colony after only giving her a couple of warnings, I'm sure Data can vaporize a man who has just vaporized someone else and is holding sentient beings against their will.
The only problem here is the lie Data tells Riker back on the ship. And it IS a lie in the legal sense, Data with held information either through misdirection or omission. Yes "something" happened during transport that deactivated the weapon but that doesn't change the fact Data pulled the trigger to fire the weapon in the first place.
Just the same, I'm sure Data is full-on in the clear for his actions on the ship and what would have happened if the Enterprise arrived moments later.