I just don't think you can say it was "Absolutely not" a bad decision. I think you can say something like, "In retrospect it was not an irrational decision but the red flags were significant enough that it should never have been made." To my way of thinking, that makes it a "bad decision."
To my way of thinking, also, his contributions "to 3 division titles and 1 AFC championship" are trivial in comparison to the damage he has done to Sport in general and the Patriots in particular.
This is a game. A game to which we take our kids. Hernandez' actions mean that, for the last several years, I've held up to my kids a team that gave haven to a guy who likely committed one murder and may have committed three. My history as a fan of the team is long enough and my admiration of the Krafts is deep enough that they will overcome this situation, but it leaves a very bad taste in my mouth.