Why he did it? Because he can. To be honest I would do the same and I don't have a tragic family history as he has. If you criticize him for doing this you don't have children or you are a hypocrite or maybe a psychopath... No, this definitely does not make him a better president, I'm just saying that I understand his action.
Discussion
Yes, he had the right do do it. That is not the problem.
However, he had staunchly denied he would ever do it, until he did it anyway. That's called lying, and in the light of his own past criticism of some anticipated Trump's pardons which have never been actually issued, it's also extreme hypocrisy. That's the problem.
Totally, I do agree with you 100%. He acted opportunistically. I'm just saying that I would do the same including the staunch denial. In his position right now the only thing which can be damaged is his legacy... So yes, he would be a fool if he didn't do it.