Apparently, after Mitchell had killed his friend Kelso in cold blood (more like "crushed like an insect" as he had announced he would in the transporter rrom) he had crossed a line and revealed that he had become personified evil (i.e. militant selfish- and ruthlessness) and considering his powers he needed to be stopped, judged (for killing a fellow shipmate) and executed.
Kirk made that clear with what he probably thought would be his last order:
KIRK: If you have not received a signal from me within twelve hours, you'll proceed at maximum warp to the nearest Earth base with my recommendation that this entire planet be subjected to a lethal concentration of neutron radiation. No protest on this, Mark. That's an order.
Of course and most likely, the Q Continuum would have put a stop to Mitchell's rampage (where do the Q come from, did they also encounter the energy barrier?) but Kirk did it himself, so they didn't see the need to interfere.