How can preventing a witch hunt be obstructing justice?

We now know beyond a shadow of a doubt that Trump didn't collude with Putin because if he had Mueller would have found out.

So we know that the entire purpose of Mueller's investigation was invalid; there was no collusion.  Which in turn means that Mueller was investigating an innocent person.

Given that the purpose of justice is to punish the guilty and clear the innocent anything that Trump did in order to end an investigation into a crime that didn't happen wouldn't be obstructing justice it would be helping ensure that justice triumphed.

Imagine for a moment that Hillary was being investigated for cannibalism based on a rumor from Russia that was dug up by a British ex-spy who was being paid by Trump.  Would anyone, including conservatives, think that it would be obstruction of justice for her to do everything she could to end what was clearly an unjust investigation?

The basis for the whole Mueller investigation was even less credible.  It was started because Trump fired Comey, the guy that the Democrats had been calling for to be fired for months.

But Trump fired Comey because Comey was lying to the American people and all of a sudden we were told that Trump must be a crook. Remember Comey told Trump that Trump wasn't under investigation. When Trump told the world that Comey refused to confirm it until he was forced to while testifying to Congress under oath.  Given that the #FakeNews media were saying Comey was investigating Trump Comey was lying by omission.

Hence the firing of Comey served justice by ending the politically motivated lying.