I'm assuming this is meant to be sort of an Abraham/Isaac situation.
The question pre-supposes I believe this thing is sacred. I don't believe in God, but if I fully, completely believed in it and that it had given me this mission for some kind of greater good or to be part of its plan, I guess I would. Over-religious people kill people all the time and I have little doubt they truly believe they are doing the right thing. If my mind were as messed up as their's, I don't see any reason to assume I'd be any different.
If you believed it was a sacred act, would you kill someone?
This is impossible to answer because "sacred act" implies that you believe it is the most moral thing to do (more moral than the immorality of killing). If we're going to assume I'm religious enough to believe such a thing, then my personality would be completely altered and it's impossible to predict how I would be thinking, considering I wouldn't be deciding anything based on logic.
If you are really just asking "would you be able to kill if you were completely convinced it was the moral thing to do", removing religion from the equation, then my answer is that it's hard to say without being in that situation. My guess is that I wouldn't be able to unless it was very directly self-defense (or possibly defense of loved ones), and even it would probably only be possible if I didn't think about it very much beforehand.