Messing with ideas

I’m lately messing with ideas about jealousy and resentment; to what extremes would a character go if they felt that they were always being sidelined in favour of someone else? If they felt that another person seemed to be making the headway that they felt was owed to them? If the balance of their mind was already disturbed, how much would it take to finally tip them ‘over the edge?’ I am enjoying exploring this darker side of an individual and get the feeling that my third novel will expand on this topic still further. Great times ahead!