More specifically, someone who might open "undesirable doors" and fill you with profound regret for letting him/her into your life.
The idiom I'm trying to find would be used in the following situations:
- A married man hesitates about going on a date with a woman he met recently and thinks: "I'd better keep away from her. She looks like a real ...................
- A new neighbour has been acting too friendly but, in your judgement, his personality looks like trouble. You decide to put up a barrier because chances are he is a ...............
Is there a metaphor for that? I'm not looking for the word "psychopath" or one of its synonyms.