I flew more than once with Julian Monaghan as my co-pilot around 10 years ago. He was a thoroughly decent regular guy and a very competent pilot. He did not drink excessively during our layovers, or rather I didn't observe or suspect him of drinking heavily.
He is certainly far from stupid and he is paying and will continue to pay a heavy price for what has happened. I would hazard a guess that he might have developed an alcohol problem for whatever reasons in his life.
I do not excuse his behaviour but I suspect it was not a reckless disregard for the rules or common sense but something far deeper.
I hope he gets the help he needs and it saddens me to see someone lose their career in this way.