Increasingly I’ve found myself shying away from the hero/villain archetypes because they paint a picture that’s too black & white. In the particular scenario you described, by thinking in those terms, it invokes the idea of “defeating” an “enemy” where the real goal should be deescalating the situation.
I’m going to speak in a hypothetical because the real me would be too much of a coward to intervene. As much as possible, it should be the goal to stop the abuse while leaving the perpetrator's dignity intact, after all, beyond the immediate situation there’s also the resulting aftermath to consider when the couple leaves. Doing so requires at least a little bit of empathy toward the perpetrator to at least express that you understand that they’re angry. This isn’t the same as condoning the action.
Where it goes from there will depend on how the person responds, and whether it appears to be a fairly isolated fit of rage, or whether it is truly an abusive relationship which is probably beyond the scope of what a bystander is able to address beyond discretely informing the victim to seek help.
At least that’s how the scenario plays out in my head.