Betrayal

The act of leaving one group and joining another one, which is the betrayed team's enemy or rival. In fiction, it's more likely for a protagonist to commit betrayal than it is for an antagonist for the sake of drama and Hurting The Audience. When the Good Guys are betrayed, it's up to them to make their best effort to bring their traitor of an ally back on to their side. That is, of course, if they aren't too mad at the Traitorous Ally to want them back. In cases of an ally joining the enemy against their will, that isn't betrayal as it wasn't their choice to leave their friends.