If you copy or gain control of a Regicide that's already on the stack but you didn't draft one, you won't be able to choose a new target and Regicide won't resolve as it tries to resolve. The target creature won't be destroyed. On the other hand, if you did draft a Regicide, use the set of colors chosen as you drafted cards named Regicide when choosing a new target (if applicable) or determining if the resolving Regicide's target is legal.