You cannot see Christ in a truly evil person, because Christ isn’t present in him or her. To attempt to see what isn’t there will drive you mad, so don’t try.
However, I am not sure if this applies to the woman you speak of. If she is simply very annoying, rude, or otherwise unpleasant in her ways, that is not at all the same as her being evil. If you have good intuition, you can probably feel the difference. An irritating, annoying, unpleasant person, will make you feel angry or frustrated. But an evil person will make you feel afraid, not by doing anything in particular, but because you will sense the cold emptiness in their heart. (You mentioned cruelty and sadism – these could be signs of real evil.)
And there is a third possibility: the woman may be “average” in terms of ethical quality, but affected by demons. This isn’t nearly as bad as being totally evil, but someone like this still has a “scary” vibe around them and will make your intuitive alarm-bell go off.
Try to assess which of these three she is: evil, annoying, or affected by demons. (Evil people are fortunately pretty rare. Annoying and demon-infested people are common.)
As said, if she’s evil there’s no point in trying to see Christ in them. If she’s demonized, Christ may be present in her, but appealing to Him will probably trigger her demons. If she’s just annoying, then it’s probably more of a personal thing between you and her, and she may well be just as much a child of God as you are.