As we are seeing right now, in a climate like this one, a lot of people can get caught up in something and whether or not it's legal, bad things can happen to them. I can't read McGahn's mind, but the source of the problem is that there's some real ambiguity in the law, such that it's hard to say when perfectly legal interactions become illegal interference. McGahn didn't want to even go near the grey areas.
However, what was really happening with the McGahn situation was that Trump thought Mueller was on a "witch hunt". Trump thought Mueller was biased and was being unfair. He thought the investigation was a waste of time and money. Now, in the course of this investigation and the media coverage thereof, we have heard it said many times that no one is above the law, not even the President of the United States. I want to flip that around, and note that no American's constitutional rights can be denied, not even those of the President of the United States. How could President Trump express his opinion that Mueller was off the rails? What would it be legal to do? What he actually did was to tell his lawyer that his lawyer ought to talk to the Deputy Attorney General, and tell that official that it was the President's opinion that Mueller had gone too far, and that the Deputy Attorney General ought to replace Mueller with somebody else.