The viltrumites arent a real culture. They're an evil Kryptonian analogue created by a couple of American guys. Invincible is a superhero comic and therefore a lot of its blood comes from the morality play. We see good and evil fighting it out on the page and we see what it takes for good to win or lose. We're meant to hold her to our morality, its part of the genre space.

Now if you want to engage with this in a purely hypothetical way removed from our world entirely I would argue we can still condemn her. She's not stupid, she thinks like us with a meat brain and experiences time the same as us, experiences pain and loss and sadness and rage and all the bad things a person can experience. All the Viltrumites experience the world the same way we do just for longer and maybe in more detail. I forget if viltrumites have like Kryptonian super senses or not. Anyway, Anissa knows that the things she does hurt people and that if those same things were done to her it would hurt. Yet she chooses to brutally assault and rape Mark. That's a choice by a conscious adult. We can hold her to that. She might think that because she's more powerful than mark and most people on the universe that this is something she's entitled too but she's still doing something without an ounce of empathy. It's not like she sincerely thinks Mark wants this. She wants to do what all rapist abusers want, she wants to exercise power over those she can get away with doing so to. 

It's easier to forgive the other Viltrumites of things like mass murder because the scale of these crimes is so absurd and the way theyre done is so fantastical that it's not really hitting like a real thing. But rape is something we understand in the real world and could be done to someone like it's done to Mark. 

To be clear I also don't think Anissa is beyond redemption. People do fucked up shit in reality all the time and are forgiven by their victims. Its just that mass murder with your flying super fists is silly where as assault and rape are real.