Karkat's eyes widen in surprise as soon as it becomes clear what the supposed human is doing, but by then it's too late – he'd assumed the man would get exasperated and drop it and go look for a different vehicle to mangle, but he was wrong.

He squawks in pain and surprise as the holomatter fist makes contact with his gut, and stumbles backwards, falling on his ass; within seconds he's scrambling back to his feet, though, and though there's a wheeze in his growl and he's visibly wincing he bares his fangs and snarls. The human started it. That means anything he does now is justified. He knows the rules.

Karkat tackles Starscream's holoform without further hesitation. He doesn't have any weapons, but he does have fists and claws and a righteous anger.