User blog comment:Gundam Legilis/Viewer Discretion/@comment-25110963-20160102040117/@comment-26397825-20160102042958

It's implied that Mercury hasn't had a good relationship with his dad. His dad is a drunkard, as evidenced when Mercury is describing Qrow to Cinder, and I got hints of dislike there. Then he is all beat up in the sneal peak, Cinder and Mercury are outside alone, and there is a flickering fire off to one side. It's Mercury's recruitment stage, so there must be a reason for him joining.

The way I figure it, his dad was a drunkard, went too far one day, Mercury retaliated and killed him at least by accident even if not on purpose, and the house caught fire. Cinder saw the aftermath, compliments him on his strength, and he says he's not strong enough. Cinder offers to make him stronger and recruits him to her cause. Voila. Mercury is now a bad guy.

It's somewhat cliche, but it's a legit reason for the settings we have seen so far. Of course, I may be completely off base with all this but it's a theory. We'll find out tomorrow/in two days.