For my games, it is that they kill in COLD BLOOD. They are paid to take out someone, period - they do not care if that person is good or evil, lawful or chaotic, worship X or Y; they just kill because they are paid for a life.
A sniper is not an assassin, he is a speciality warrior during combat, his job is to take out the enemy. Warriors are not evil because they are fighting and killing for a reason and for the most part only fight other combatants.
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In a game of mine, a player would not kill anything, unless he was paid. Orc, goblins, demons, drow, he would let them attack the rest of the party until they agreed on fees.