Season 2 episode 7, Anissa kicked his sorry ass and gave him the pitch: take over your father's mission and conquer Earth, or we'll kill you and do it anyway.
I think I'd give up and do it honestly.
Obviously, that's not really what happened. Mark was right to resist. But GIVEN WHAT HE KNEW, was that really the smart decision?
You're a teenage (half-)Alien, you've had your ass kicked within an inch of your life twice now by grown adults of your own species. You're only alive because they chose to let you live, and this was your last chance.
For all you know, there is an entire army of them. An entire species. You don't know there's only a few dozen of them, but even if you did there's no reason to think you have even a shred of a chance of winning.
If it was JUST that simple, "conquer Earth or die and we'll do it anyway", I would choose to die fighting them. Because I'm not willing to do something so evil just to save my own ass.
But it's not just about me. The Viltrumites they'd send to replace me would be a whole fuck of a lot meaner and impatient than me, if I was put into this position I wouldn't be able to think about anything else but how many more people they would needlessly kill compared to if I was in charge. Even I too would have to do horrible things.
I would think about how if I could get the GDA and world governments to give in quickly, this would all go smoothly and maybe everyone could go on to live lives worth living in the least. If I'm to believe there's even a tiny shred of truth to Nolan and Anissa's promises of health and advancement.
I would think about how my replacement(s) would, if I'm to learn from Nolan and Anissa's habits and what I've heard about Viltrum, let the streets run red with the blood of civilians. How they'd make examples out of surrendering rebels.