Every now and then, we could read in news that an 'always-warm-hearted' man is actually a murderer. For another example, a great contributor in stackexchange who has devoted himself/herself to the community, might turn out to be a child molester at large.
If a person is kind in some part of his life but a devil in another part, i.e., he/she leads a double life of Jekyll and Hyde. What do you call this kind of people?
The problem with the other answers so far is that they don't describe a person who displays two conflicting qualities at the same time.
A 'wolf in sheep's clothing' is someone who presents themselves in a false way to hide their true intentions - seemingly benevolent characteristics are actually part of their malevolent plans, not a separate facet of their personality. Likewise, a 'Jekyll and Hyde' character is someone who changes their personality so starkly that they are like two different persons. It can describe someone who has violent mood swings, quickly changing from calm to angry, for example.
What you seem to be describing is someone who has contradictory qualities - they display good in some areas of their life, but evil in others.
You might say that such a person had "a dark side to their personality". Another expression is to say that they are "a walking contradiction" (or simply 'a contradictory person'). I've heard a number of screenwriters use the phrase "Hitler loved dogs" as a way of stating that fictional characters, like real people, are not always easy to categorise, and that no single person is either all good or all bad.
This is a common expression that means someone appears or presents themselves as kind, or at least quite benign, but in fact is a predator.
Merriam-Webster defines it like this:
: one who cloaks a hostile intention with a friendly manner
He is a "Jekyll and Hyde" character. This story of a man who splits his personality into a respectable and an evil part is well enough known for it to be an idiom to describe someone who seems pleasant but is sometimes evil.
James Littlehales said: “He is very much a Jekyll and Hyde character and he is a nice and pleasant man when sober and drink and drugs can turn him into the opposite.”
