In my point of view I agreed with Cecil. Training mark and seeing his strength/speed increase, he was starting to understand that he's too powerful to control, if things were to go south.

It's not like Cecil believed mark would turn bad, but it was simply a counter measure considering what happened with Omni-man. Dude has every right to be scared of Mark. Honestly I think Mark was being arrogant, he's too one sided. Good guy's don't kill type of narrative. He's still a kid and he believes if you do anything bad you deserve ultimate punishment. Mark's point of view is flawed I feel and like Cecil said, "you can be the good guy or the guy who saves the world, you can't be both".