I think that Gautum saw Nakul kill Tripty, and he took the knife from him to shift the blame on himself. He is Nakul's father after all. It is also possible that Mahir was with him at the time, and he is keeping quite for both there sakes!!! Gauthum is his son after all!!!
Another Point:
When Tripty was was pushed down the stairs to her death, everyone was shown running towards her but Mahir, yes they say he was sick!! BUT WAS HE REALLY ACTUALLY THAT SICK!!!! these are some points i wanted to bring up! what do u guys think?????????????????