Is it ever ok to resort to playing dirty in order to get the upper hand?

I have indirectly encountered a couple of situations recently where someone could easily obtain the upper hand if they decided to play a little dirty.

The first example: A man who is leaving his baby mama and knows for a fact (and this is a fact most people familiar with the situation can attest to) the baby will be better off with the father having full custody. This is in a state that tends to heavily favor custody with the mother. The father knows a few things about the baby mama that could virtually ensure that he gets custody but would mess with this lady’s life in a very, very big way.

How about in a situation where someone is attempting to rip someone else off? The person being ripped off has a card up their sleeve that will turn things around on the person trying to rip them off but will ultimately cost the scammer much more financially then the scam would have ended up costing the scammee.

None of the individuals involved in either of these situations is a grifter, they are all just ordinary people trying to get by and looking out for number one. Is it ok to resort to playing dirty in either of these situations? Is it ever ok to play dirty?