Seriously. People who think he should have gone after the guy (physically, not legally) are idiots.
I speak from experience: I am an idiot. I very likely would have started shit with the guy, and I would have made things way worse. I'm a confrontational person, and my temper escalates from zero to fury in seconds when I see people being victimized.
When I'm outside of the heat of the moment, I know it's the wrong way to handle it, but when it's going on I feel like, "fuck this guy, let's see you fuck with me, you wanna fuck with someone so bad." And yeah, I have good intentions, but in a situation like this, which is already traumatic enough for the poor woman?
No way.
Here's what would have gone down:
Upon waking up:
"Wha- Yo, what the fuck are you doing, asshole!?"
Probably he stops attacking the woman, probably she recoils away from him, distraught, anybody within earshot looks at me and the guy, probably he says "nothing..."
"The fuck nothing, I saw you assaulting that woman you piece of shit, fuck you!"
More people would be looking now, and hearing what I'm upset about, they'd now be looking at the woman too, who likely wouldn't want any of this attention on top of the shit she just went through.
If the flight attendants weren't already on their way to deal with me, I'd probably try to get a hold of the guy physically at this point.
From behind I could choke him out easily, but I wouldn't want to do that, I would want to make him feel like a little bitch, so I would probably need to leave my seat to move into the aisle to lift him up out of his seat (probably by his collar/shirt/jacket) and get in his face. Choking him out would defeat the purpose.
Around this time, I would definitely be getting restrained, and if there was an air marshal on board I'd be arrested about now. Even if they listened to my story and restrained both me and Captain RapeDouche, my next several weeks or longer would be fucked, and the guy would walk because my testimony wouldn't be enough to do dick, and the woman he attacked would have already been completely embarassed by my actions, so she wouldn't want to be dragged even further into it, and the RapeDouche would go free, and assuming he's as much of a douche as we have reason to believe, I'd wager he'd press assault charges on me.
I realize there are plenty of keyboard warriors out there to claim they'd have done this or that, but the reality is that whether or not this douchebag deserves to be assaulted in a rough equivalent of what he's done to his victim, ain't nobody can do it without simply making things worse for the victim...
Unless you followed him out the airport and bashed his fucking face in somewhere dark and obscured...
Still. OP handled things about as well as they could be handled, given the fucked up situation it was. There was no vigilante justice, or any real justice of any sort, but he saved the woman from things getting any worse than they already were, and he did so without endangering her to the douchefuck, or without making it anymore emotionally/psychologically traumatic.
Justice for the douche would be nice, but not at the expense of making things worse for the victim.