Actors. What are the signs of a extremely dangerous person who committs to a serious relationship-to destroy

Question

the other people and their whole family. (financially, mentally, physically?)

Answer 1

Someone who has incredible selfishness that is not tempered with morality and ethics and has a reason to do something like that (anything from theft, revenge, or even just for the heck of it).

But, there are people who can do that while being incredibly deceptive; they may actually appear to be genuinely nice, friendly, and considerate; they can wear a mask frighteningly well and can lie frequently but also very convincingly.

That's psychopathy in a nutshell.

A criminal psychopath is frightening because they not only commit terrible deeds, but are also completely cognizant and calm about it as well.

This is in contrast to a psychotic, who does something terrible but may not be aware of what he/she is doing.

Basically, it would be a performance (or just the story itself in the eyes of the audience) that can convey a calculated and deliberate evil intent to the audience while not showing it to the other characters (of course, for that to work, the audience must feel for the good characters and must feel involved with the story).

Or, you could even catch the audience off-guard about it, like making another character out as the bad guy but then suddenly revealing otherwise.  

This depends on what the story is trying to achieve.