Where it belongs.
Mueller did his job.
He might be irked that Congress doesn’t seem up to the task, but I think that he’s satisfied with the job he, himself, has done.
His testimony will be dull.
He won’t go beyond quoting his report.
If pressed he may lash out at the poor Congress-critter poking the lion.
I agree that the report stands as a complete account of a thorough investigation, and provides enough detail for Congress to decide (with the help of legal council) whether to accuse the president of a crime and try to make him accountable.
The only point I can see in having Mueller testify, is with regards to assessing if Barr lied or mislead congress.
I do wonder on a few things.
1. If they go through the impeachment process and show in convincing fashion that the President committed a crime, would the Republicans be willing to hold him accountable or would they protect him at all costs? Essentially, are they happy having a leader who operates illegally, and do they support this behaviour?
2. Would the voters who are Republican supporters care that this party supports criminal behaviour, or would they then decide to no longer support that party. i.e. Will the Republican party, support of a criminal leader and criminal activity have any enduring effect on the reputation and future prospects of this party/brand?
3. If the Republicans in the Senate decide to ignore the impeachment, does this mean that Trump is let off the hook? Would it still be possible to put these criminal charges onto him once he eventually leaves office?
4. If they don't try to impeach the president, does it make it more likely that he will meet his due under the Justice System once he eventually leaves office?
ALSO, by the way, it seems Trump and Barr's strategy going forward is to attack Mueller's credibility. Trying to start an argument about whether Mueller should have made a criminal judgement and hence not making that judgement makes Mueller out to be weak and incompetent.
They are publicly decreeing that Mueller's position "It would be unfair to claim criminal activity if the president has no means to make his case in court" as being a position of weakness and incompetence, rather than one of high integrity, professionalism and compassion.
They see integrity, professionalism and compassion as being weakness.