Reasons:
1) Mihir is not that strong that he will go kill someone
2) If he is was the killer, he would have accepted his crime like a brave man and not hid behind Karan
3) He loves Karan, he is the father of Karan and if something unfair is going to happen to your own child, you would stop that and if Karan was taking his place, Mihir would never accept that.
Those are the Top 3 Reasons among others.
Please add more if you think of some.....