Mr. Laser Beam wrote:
Decker may have become unhinged, but his intention was only to destroy the planet killer. He didn't violate the Prime Directive; he didn't murder innocent Enterprise crewmen; he didn't imprison Kirk and company against their will. Tracey wanted power; Decker wanted to finish the job and atone for the deaths of his crew.
In fact, up to the point where he knocked out the security guard and stole the shuttlecraft, Decker didn't even do anything that violated regulations. He was, as Spock agreed with, entitled to assume command of the Enterprise
. And was within his rights, as commander, to order an attack on the planet killer. That may have been a mistake in judgment, but it wasn't against the rules.