I can tell this by first hand experience. I completed/earned this achievement in multiple accounts. If you use the tranq.rifle and you end up killing a target, as long as you get "Merciful soul" bonus you're ok. I once did a complete playthrough and this happened a multiple times and i still got the achievement. (Including killing that same guy you did with the T.Rifle) But if you dont want to carry the doubt with you untill the end of the game i sugest you save often, and use the stun gun. Another advise i can give is, and believe me this can really screw up the achievement. If you stay in a area for too long, and put all the guards to sleep, and if you move them after a while they might end up dead. If you want to move them make sure they are still sleeping after you do it.
Last edited by II PenanCe; 09-25-2013 at 03:55 PM.