Decker's horror at the mistake of sending his crew to the soon to be destroyed planet, and then grabbing the opportunity to commandeer the Enterprise and to get back at the planet killer is done so well - and its the little things: the Queeg-like juggling of the tapes; the disgust at having to order the Enterprise to veer off, after Spock pointing out he has to or be relieved; and the blank stare when talking to Kirk and Spock while awaiting his doom in the shuttle, and then waiting for the contact with the planet killer - girding himself, almost not being able to wait ANY longer for the release.
Yes, I've always really liked Windom, but it threw me when I heard a few years ago that Windom had no respect for Trek, and intended his performance to be hammy and over-the-top. Then I look at Decker in the shuttle being swallowed by the Cornicopia of Doom, and I start to see it. His expressions of fear start to look ridiculous to me, involuntarily, because Windom said that. I wish I'd never seen that quote. Maybe I'm too impressionable.