well...kunu will be jealous...he will get drunk...and in his drunken state he will confront siddhi and might try to get intimate with her if she tries to brush off his questions...he will be angry...possessive...as well as passionate...romantic...heartbroken...all the emotions mixed while confronting siddhi in his drunken state...
i don't think he will do anything to abhay...maybe he will react the way sujal reacted seeing piyush with kashish and he confronted her in rain...but he bad-mouth kashish infront of piyush which triggers him to warn sujal...but kashish understand how hurt sujal was...piyush-kashish left...and sujal cried...it was a classic scene...rajeev made me cry...the same might apply with kunu seeing abhay-siddhi together...he can tolerate anything...but he can never tolerate loosing his love and seeing her with someone else...