The guy who snipes could've also wittnessed how the player next to you killed someone and not IDing their body, thus he'd be about to kill a T.
So yes, unless he is shooting you, you have to wait until he killed the guy next to you. And if the killed person was an Innocent then go ahead an declare a KOS them.

But how you described it it sounds like you joined a gunfight/killed on suspicion.

The guy who snipes could've also wittnessed how the player next to you killed someone and not IDing their body, thus he'd be about to kill a T. So yes, unless he is shooting you, you have to wait until he killed the guy next to you. And if the killed person was an Innocent then go ahead an declare a KOS them. But how you described it it sounds like you joined a gunfight/killed on suspicion.