I disagree. Although the player at hand may not have a good justification for multiclassing this way, that doesn't rule out the possibility that a creative player can justify it in an interesting way. In fact, a character like that could be very interesting. (Although also munchkinlike in many ways)
As to the topic at hand, I don't believe that extracting vengeance on a group that killed your friends is an evil act at all. It is merely using your stealth to your advantage. If the PC CdGs an evil sorceress queen while she is sleeping, is that an evil act? I would say no, because you would be really nasty if you woke her up and challenged her to a fight. Granted a Lawful character should have qualms about CdGing while the opponent is asleep. But chaotic is chaotic, and it is my experience that players who declare their characters CG often roleplay NG instead. Chaotic characters disregard laws, traditions and the such, they concentrate on achieving what they see as their goals, not on making sure that everyone has a fair chance.