Why Is Donald Trump Hellbent on Dismantling OPM?

Last year the Trump administration unveiled its plan to reorganize government. I think every president puts together a plan like this, and it was the usual collection of good ideas, bad ideas, and fantasies that would never get through Congress.
One of the proposals was to reorganize the Office of Personnel Management. Part of this was uncontroversial: everyone agreed that it was a good idea to get OPM out of the business of background checks and instead move this function into the Department of Defense. That’s already a done deal. This left two things:

Kill off OPM as a separate agency and make it into a department within GSA.
Move its policy shop into the White House’s Office of Management and Budget, where it would report to the president.

It’s not really clear how this would help anything on an operational level. The boss of OPM would get a new boss, and that’s about all. On the policy side, however, it would continue the process of consolidating ever more power into the OMB, where the president has tighter control of it. Even Republicans were unsure that any of this was a good idea, so the planned change hasn’t yet gone anywhere. Today, the Washington Post reports that Trump intends to play hardball over this:

The Trump administration is threatening to furlough — and possibly lay off — 150 employees at the federal personnel agency if Congress blocks its plan to eliminate the department. The Office of Personnel Management is preparing to send the career employees home without pay starting on Oct. 1, according to an internal briefing document obtained by The Washington Post. The employees could formally be laid off after 30 days, administration officials confirmed.
….[Margaret Weichert, the acting head of OPM] has told her staff that she is “planning to play chicken with Congress,” according to three officials familiar with the comments.

The alleged reason for these furloughs is that OPM used to make money on background checks, and since they’re losing that money they’ll need to get rid of some people. However, Congress has already agreed to make up this revenue loss, so that’s not the real issue. Nor is it plausible that anyone in the White House cares all that much about whether the government’s health and retirement plans are managed by a standalone agency vs. a department of GSA. So what’s the real reason the Trumpies are fighting so hard over this?
The only plausible rationale still remaining is the move of OPM’s policymaking function into the White House. Apparently Trump really, really wants direct control over personnel policies, and he’s willing to go to the mats over this. But is that a good idea? If another president had suggested it, I might shrug and figure it’s just part of the steady centralization of executive power in the White House. Trump, however, has made it pretty clear that he doesn’t like it when he’s not allowed to hire and fire whoever he wants, whenever he wants. More control over civil service policy might be just the ticket to eliminating anyone who might get in his way.