The second sentence is key, though. It seems to temper the conclusion that Trump would be firing Rosenstein or Mueller solely for his own purposes. It suggests Trump is worried about what firing Rosenstein or Mueller would do as far as prolonging the investigation — not so much what it would mean for him personally.
But even if that's the case, the decision would be dependent upon Trump's level personal jeopardy. If there is a good reason to fire Rosenstein or Mueller and Trump is deciding not to do it because he's not in trouble, that suggests the decision is at least somewhat about himself.
“As it relates to the DOJ investigation specifically, the statement, if proved by evidence, does help a possible future case against him for obstruction because it shows that at least part of why he does whatever he does regarding the investigation is about doing what is best to protect him,” former federal prosecutor Patrick Cotter said.
And it would be one of Trump's most direct (known) comments to date when it comes to whether he's meddling in the Russia investigation for his own purposes. The big quote on that front has long been Trump telling Lester Holt in May 2017 that the Russia probe was on his mind when he fired FBI Director James B. Comey. Similarly, Trump suggested in a tweet earlier this month that what some people viewed as potential obstruction of justice was just him wanting to “fight back.”
....doing things that nobody thought possible, despite the never ending and corrupt Russia Investigation, which takes tremendous time and focus. No Collusion or Obstruction (other than I fight back), so now they do the Unthinkable, and RAID a lawyers office for information! BAD!— Donald J. Trump (@realDonaldTrump) April 11, 2018
Both of these were highly suggestive that Trump was acting in his own interests when he took action related to the Russia investigation, but Trump didn't say it so directly. In the Holt interview, for example, Trump didn't specifically say he fired Comey because of the Russia probe — a point I think people overlook. Legally speaking, that distinction could matter.
“This evidence would bolster evidence like the Holt interview and Comey’s evidence about Trump’s statements to him,” Cotter said.
And if nothing else, the newly reported Bloomberg comments reinforce this central fact: A president who has been pretty careless with his public comments in re: potential obstruction of justice is probably even less careful behind closed doors. To the extent that he has said stuff like this and his current and former aides are telling Mueller about it, there could be a bevy of potential obstruction-y comments we don't know about.