Perhaps they were monitoring the situation and decided to 'allow' Mitchell to be killed, because if he escaped (which he surely would), he could threaten even them.
It's a bit silly of Spock to conclude that just because Mitchell's abilities grow exponentially they will eventually be infinite. So what if he's superior to you now, Spock, and continues to get better? That doesn't automatically and categorically make him infinitely powerful... Q powers might have been well beyond what Mitchell could at best/worst become.
Gary could easily have made the ship blow up, or crash into the planet. He had to be stopped, period.
But if he could easily make the ship blow up or crash, how
could he be stopped? Hitting him with a fist would be the least likely way to succeed!
Kirk's choice at this stage should still have been to go for the most potent weapon in his arsenal: the radiation bombardment from orbit. Toying around with feeble phaser rifles put everything in jeopardy.