I'm not sure if it's Oliver's disregard for human life or impetuousness of youth, but he certainly knows how to make the wrong choices. I would think he would be a bit more wise considering his accelerated development, but I guess not. On the page where he punches Mark and calls him weak, I thought he was really embracing his Viltrumite heritage/roots/DNA. Thragg showed all kinds of awesome, despite being a planet conquering despot. It's not so much the restraint he demonstrated as it was the wise arguments he gave Allen. He did the same with Mark when he first came to Earth and negotiated their truce.
Generally, powerloss stories get on my nerves, but this looks to be interesting. Thinking about how Viltrumite DNA works (as described in the Handbook), Mark's powers will return. I wouldn't be surprised if his human side plays a factor in it.
Final thing: what must a Viltrumite have to endure to become as powerful as Thragg? Allen bulked up after his near-death experience. Thragg must have been taken there numerous times.