Let's do a character analysis.
On the one hand, we have a player who threw away chances in college based on misbehavior and terrible attitude and was working at a fast food restaurant when he got a last chance, was signed as a UDFA, and made good for a few years and was a top player for us before he started to feel disrespected after recent contract talks went sour and after that the attitude problems started to resurface in the last year or so
Versus
A man who for 18 years has been doing what he felt was best for the franchise and its ownership and their best possible chance of winning Superbowls with a single-minded, almost religious devotion to that goal above almost all else.
It is entirely within Butler's character to backslide into the guy who blew his first couple chances at making a life out of football and wound up a UDFA despite what should have been second- or third-round talent.
It is entirely within Bill Belichick's character to size that situation up as it's happening and make the football decision that Butler's deteriorating character does not allow him to be a viable option in the Superbowl and decide that putting the best possible team on the field is more important than putting the best possible jersey names on the field, and that Butler is not part of his best possible team anymore.
That would be technically a football rather than a disciplinary situation, it would fit both the character of the two men and all the available facts. Butler backslid to the behaviors and personality that got him kicked out of college the first time round, and Bill decided that he couldn't trust him to perform at the level we'd come to expect, and that he couldn't risk putting a player he didn't trust on the field.