At first I incline to say it's omni sage for the other dude but since you mention killing its possibly be the dude who led the assassin organization who is also an S (I forgot his name)I honestly don't think she's a cultivator. When the MC asks Xiwen what he would gain by choosing sides, Xiwen replies that, since the MC is a cultivator, only two people can guide him, one is his grandfather and the other would kill him immediately. In the line in question, Xiwen clearly says "he", meaning that, in the world of POMA, it can't be a "Lady".
I don't believe that Xiwen would lie or hide facts, especially considering that just before this he had already mentioned Lady Butterfly.