This is making him physically ill. He has to do something.

He doesn't lay a hand on her weapon -- that would be a challenge, and he has no intention of even hinting at anything like that. Karkat doesn't try and hug her out of comfort -- that would be too familiar, too intimate even for such a simple little thing. He has to be careful here...not just for his life, but for personal comfort. For all his posturing, Karkat's limits were very defined.

His hand slips over her's, the one tangled in her hair. He takes a short breath, realizing that he has no idea if Feferi is going to retaliate against him for his boldness, and begins to attempt to siphon her frustration from her. The skin contact alone makes his muscles tighten with shared emotion, but it seems he is much better at keeping it contained outside of minor visual queues.

He says nothing -- nothing needs to be said, and he needs to concentrate until he feels her relax. Karkat clearly makes no plans to stop trying to thieve her negative energy until she's back under control of herself.