It is telling that the White House Counsel put the kibosh on firing Mueller.
The job of the White House Counsel is to basically say "why yes, Mr. President...of course that's legal" and then set about trying to make the case. Remember Bush and torture? Despite the fact that torture is clearly illegal, the White House Counsel called it "enhanced interrogation techniques" and then made a case as to why it was perfectly legal to torture people. If this report is true, then what McGahn did is wholly out of character for someone in that position.
Anyone who understands solicitation and the federal definition of conspiracy would have told Trump to fuck off.
Trump solicited McGahn to commit a criminal act. Here, the criminal act was Trump soliciting McGahn to obstruct the investigation (obstructing justice). McGahn knew that. For McGahn to have carried out the order would have then created a conspiracy (an agreement between two or more people to carry out a criminal endeavor with more than a mere preparatory act in furtherance of the object of the conspiracy--this part depends heavily on district, but any definition of conspiracy is applicable here). By disobeying Trump's order, McGahn insulated himself from culpability in Trump's criminal act of solicitation.
The asking and/or encouraging someone else to engage in a criminal act is a criminal act in itself provided the intent is to achieve the object of the solicitation. We have both elements of solicitation here.
That Trump was too big of a pussy to carry out the act himself is irrelevant.
Committing crimes in office: MAGA!