So Teenage Kate has an abusive boyfriend, the Pearson's go to the cabin because Kate is in danger and find out boyfriend was extra abusive (not physical, but this dude is seriously emotionally abusive) to her, and mom just yells get out of here and the brothers stare blankly....
Sure, Marc seems like the type of dude that would report it to the cops.
His slow swaggy confidence in leaving after Rebecca told him to get out is proof of that.
And yes it would have caused problems for Kate as she would have had to explain,
"Oh, please Mr. Police Officer, don't arrest my brothers as they were only trying to protect me."
But the fact that when Marc left, he wasn't afraid of her brothers is a problem.
I know the people at This is Us listen to twitter comments, and I predict that in the future there will be a 60 minute episode of the brothers beating up Marc, LOL.
I'm pretty sure that's not the last we've heard of Mark. Got off a bit easy there, and apparently there was some serious trauma based on that scene 20 years later when they find his picture. What he did that weekend was bad but not lifetime damage type stuff. I bet he tries to get revenge or something.