That would depend upon the definition of "evil" in your game world.
Yes, but last time I checked, modern day snipers don't appear in D&D.
This isn't being dismissive - the D&D world is fundamentally different from the real modern world. In our world, alignment doesn't exist. In the D&D world, alignment is a force of the universe that interacts with magic. Define how that force of the universe works in your world, and the rest falls into place.
Just to expand on this point for a second.
Game world morality is not based on any sort of cultural belief system. Game world morality is a physical reality. Good just isn't some sort of philosophy, it has just as much real existence as gravity or electricity. It's 100% objective.
The game world morality has decided that a particular set of actions is always evil. Spells make the best example of this. Animate Dead, no matter what you actually use the spell for, is an evil act. Always. I could animate skeletons to save orphans from a burning building, and I've still commited an evil act (note the evil act is the spell, not saving the orphans).
Now, they, the game designers, have decided that assassin=murder. Murder=evil. Therefore, assassin=evil.
A sniper, in your example, isn't an assassin because he's not evil. I know that's a pretty facile example, but, trying to apply any sort of depth to D&D morality is a morass that you will never dig your way out of.
Also note, that just because a character is evil, does not make him incapable of performing good acts, or even being a hero. It's simply the way the universe describes that particular character. The sniper could be an assassin (and therefore evil) yet be a pretty nice guy, good with his kids, pays his taxes on time. He just belongs to a class which defines him as evil.