Like i said before I too didn't kill anyone just to get a no-kill achievement but then I realised that I DID kill O'Malley and I was using that save-game file for the playthrough. xD All my dreams were shattered! lol.
Then again I played the game but this time at the end you have to kill/knock a guy at the sewers. He has a heavy gun.. I punched him, he fell into the electrocuted water(which I later realised, killed him). I didn't pay much attention and completed the game without killing anyone. BUT STILL I DID NOT GET THAT ACHIEVEMENT! !@#$% I really wanted it. I gave up. As for my inventory I only had a fully upgraded pistol and a shotgun. The boss fights had enough weapons in the rooms themselves so I didn't need to prepare for them beforehand.