A common reply that we get if we try to tell others about is that its none of your business. Allow me to give you an example where the it would not be my business but you will admit that I should have interfered.
Suppose you have a child of about 3 years old. You go with your family to a far away place. You and your spouse are busy setting up the things for a picnic. Suddenly, you notice that your child is missing. When you look around, you see that the child is walking along a dangerous path and will soon come to the point where he will fall over the cliff. You see me standing just near the end of the cliff. You shout for your child to wait but you know that it is to no avail. You see me smiling at the child. The child passes in front of me and eventually falls over the cliff.
Will you or will you not blame me for the death of the child?
Of course you will but I can say that it was none of my business!