Guys, I think that KS has black-mailed Arohi to marry him by saying that if she doesn't reject Arjun then he'll harm him by killing him. Arohi then will have to reject Arjun to save his life.
Arohi might be going to meet KS to say that she has rejected Arjun & now KS shouldn't harm Arjun & Arjun comes to know about it. Then Arjun & KS have a fight & Arohi kills KS in defence of Arjun but Arjun takes the blame & then comes the OM moment.
This is just my thought. I might be wrong.
Do give me your views on this guys.