Mitchell already showed signs of being content with building his own paradise - a conventional one, but possibly a first step in creating a whole separate universe for his personal purposes. But that probably wouldn't stop him from blinking other universes on and off just as a hobby project...
I think Kirk always knew that Spock was right: that Mitchell had to die. But it took a lot until he would admit to this. Grabbing a rifle and going after Mitchell was probably still a half-measure - something that Kirk excused to himself as a "rescue operation" to save Dehner's life, but that in reality was just a final attempt to delay
putting Mitchell to death. The rifle still had a stun setting, probably, and at least allowed Kirk to have dialogue from behind the sights; the orbital irradiation would be final.