well, kul thought simran died on the spot because of the accident so he killed sumer out of anger. but yes, he shouldnt have taken that action out of anger. but both are responsible to an extent becuse sumer should have cleared everything when the rishta was being fixed rather than waiting till the last moment. so sumer and kul are both equally responsible.
but i am worried that when he finds out abt yug and vrinda, he will not let vrinda marry sumer's son...