Probably not. There have been people pardoned before charges were even brought (such as Ford’s pardon of Nixon). And while there is a legal theory that admission of guilt is a component of accepting a pardon, it’s disputed.
That said, the states and the feds would be using the same evidence, and if the evidence is sufficient that a federal pardon would be needed, he’ll probably be convicted at the state level, too.
The precedent for this is Dzokhar Tsarnaev, who was sentenced to death under federal law for the 2013 Boston Marathon bombing. Massachusetts is also strongly opposed to the death penalty, but that didn’t help. So far he hasn’t been executed, and upon one of his appeals the case was remanded to the trial court, but this was on grounds of juror behaviour, not state law.
Plus the guy who shot the health care exec. He’s been charged both federally and by New York.
Faces the death penalty under federal charges, but not under state charges.
Our Attorney General Keith Ellison was a close personal friend of House Speaker Emeritus Melissa Hortman, so I doubt he is likely to spend much effort to protect her assassin from punishment.
(And Keith himself was on this guy’s target list!)