WHY IT IS AN EXCUSE TO SAY IF YOU DON'T DO A BAD THING SOMEBODY ELSE WILL SO YOU MAY AS WELL? 

“You should take his wife away from him for if you don’t somebody else will”.
 
We may find ourselves asked to do something we feel or think is bad for, “if you do not do it somebody else will”.

So, is it all right to steal for somebody else will do it?

Is it good to murder somebody who will be killed soon?
 
Maybe you are doing the bad person a favour by preventing them from doing wrong by doing it before they get the chance.

But the person wants to do it and has already sinned so letting them do it by not doing it before they do makes no difference. However, you should not do wrong to prevent wrong but do right to prevent wrong.

You are admitting that the act committed by the other person would be wrong but are claiming that this justifies you doing it yourself. This is a denial of human equality. You are saying that you are more important than the other person.

You do not even know if the crime will be committed for the future is a mystery to us. You would be committing a sure evil to prevent a possible evil.

It is better for a person to be willing to steal or murder or whatever than for you to be willing to do the same over that person.

“Calm down a bit. Remember it was not all John’s fault when he slept with your wife. She wasn’t forced to sleep with him”.

That is saying that you should resent both half and half.  If both are equally in the wrong that means that if you should resent John with all your power you should also resent the wife as much. But the fact is John does not owe you loyalty and she does.  The absence of logicality in the advice shows just how terrible it is to believe in free will. If you deny free will and change your emotions towards bad people as a consequence you will avoid being torn by resentment.