So, think of it this way. Nolan is a top viltrumite within their army. He’s an officer and considered one of the best. He found a lot of the creatures and aliens on earth to have some challenge to them. He could beat them, of course, but they’re pretty tough.

Mark spent a good amount of time strengthening himself and getting stronger. However, the majority of the bad guys and aliens he fought got locked up. He didn’t kill them. Then, his father essentially forced Mark to cause the deaths of thousands of people, literally.

Mix that trauma with the natural empathy and kindness that his mother raised him to have. He’s going to not only instinctually hold back, but he will hesitate to fight others, especially when they look human.

It takes a lot just to overcome those base instincts and start wailing on someone, let alone the trauma he endured. Unless you’re a psycho, that’s a lot to overcome.

“I know, because I’ve made love to a worm, and you’re both shit…”

You likely misunderstood something. Viltrumite DNA essentially overwrites the other DNA that that person is made up of. Mark is half Viltrumite and half human. When his Viltrumite powers started to appear the Viltrumite DNA overwrote his human DNA. Therefore he is now fully Viltrumite.

Viltrumites are also similar to how Saiyans are in DragonBall Z. If it doesn’t kill them, it makes them stronger. If they are never challenged they slowly get stronger as they age, however if, like Mark, they get their ass beat constantly, every time he recovers he gets more durable and stronger. He pushes himself harder than any Viltrumite before him as far as I can tell.