We'd have to have been in Trump's office on April 16 to know that. That's when Trump interviewed Mueller for the FBI job that Mueller had already held for twelve years - two years beyond its ten year limitation. All we know is that Trump's man Rod Rosenstein appointed Mueller as special counsel the very next day, on April 17.
It could be that Mueller was confronted with his own culpability from his 2001-2013 FBI tenure and "turned" by Trump to redeem himself and his otherwise distinguished record. That is plausible.
It could also be that Mueller was a white hat all along but was stonewalled for three years by Obama and his corrupt attorneys general. Mueller was appointed by President Bush in 2001 and by law should have retired in 2011, three years into Obama's first term. Why would Mueller agree to the two-year extension that the Senate approved for him? Was he convinced that Obama would be defeated and a new Republican president would appoint an honest attorney general? Mueller was in position to know where many "bodies were buried" and would be a valuable ally to a reformer president. In hindsight, this is very plausible.
Time will tell. Until then, I must trust President Trump.
“...Time will tell...trust Trump...”
President Trump knows he did not collude with the Russians and so is not concerned about the Mueller investigation.
I also trust President Trump. Remember, he has access to all classified info and knows a lot more than we do.