I don't like to put a label like 'sickness' on cheating, because then you are forced to feel sorry for them. You feel sorry for a sick person; you don't however feel sorry for a cheater. I am sure purposely sabotaging your relationship and then not being able to man up about it by giving some lame excuse like 'you don't know why you did it, you just did' classifies under some mental disorder though.
I have no problem feeling sorry for a cheater.
Because they are always cheating themselves. Even when they think they are getting the upper hand.
Doesnt mean I would allow them to use me or any one else I see them using, as fodder.