If some of these are answered later don’t spoiler as I haven’t read the comics

Why did they want to conquer Earth but not the Thraxans? Omniman says they’re gonna kill everyone on the planet

So Mark has been getting destroyed because he’s holding back and the moment he stops he’s able to take out that female Viltrumite. But when he was greeting wrecked by Omniman or Battle Beast he was still holding back? All it took was for mark to try to kill that lady and she got destroyed, instantly.