What does the very existence of the “pardon” DO to the foundation of the justice system itself?

Ensures that it is just. The idea in our system is to let a thousand guilty men go instead of to convict one innocent man, the theory being that the criminal will do it again and then we can bust them then. Part of that is giving the president the power of pardon. If he pardons someone who ought not to have been pardoned then we will get them next time.

So tell me why you oppose his pardon for Jack Johnson if you do.

And of course the president can pardon himself, it does not say anywhere in the constitution that he can't right?
And what does it matter, has Trump committed a crime?