Part of the reason Marco asked for backup, beyond just wanting an extra set of eyes around, was that it was the only way he knew to plan for the unpredictable state of his mind right after the hit. And as it turns out, all he needs to snap out of his unblinking stare at the thing he just did is someone else to tell him what to do.

He nods and lets Arthur take the gun away from him---he's on the verge of dropping it anyway---then takes the next few moments to gather his wits. He's not going to open his mouth again for a couple of minutes, or else he might start hurling again.