When you already know you won't get a conviction, there's no point in pressing forward. That fix is in and there's nothing Barr can do about it.
The Hillary decision far preceded Mr. Barr. While I do not give him a pass, I can swallow the whole "move on" concept since it was already there. I also get that prosecuting her would take out a lot of the government's infrastructure, as it is impossible to think that she e-mailed and received e-mail only from herself - those on the "from" line, and those on the "to" and "cc" lines are all equally culpable for the crimes she committed, and they're likely to be the past president, many previous Cabinet-level officials, many current and previous leadership in Congress, in the CIA, in FBI, in DOJ, in State......taking them all out concurrently would be devastating to the stability of our government. Bad, but stable, is far better than empty and unstable. We've said that for many foreign leadership with whom we've worked, and it's true for us, too. The fix must be slow. I get that.
For Comey, that was directly under Barr. Comey's tail is likely far smaller, and actually more controllable to remove - a large tumor, so to speak. He
could have merely accepted responsibility for the small list of crimes of which the public is aware, and be done. It's unlikely he was willing to do that. So, they had a choice of kill him or let him go, and they chose let him go. For all I know, he threatened them with a dead-man's switch (information to be released only upon his untimely death).
I'm far more concerned with Barr WRT Comey than Clinton, and both really suck bad to me, even after all stated above.