You've put him on a diet?
Well, that's probably why he's pissed off with you, tbh.
And really, you waited until he was nearly done before glaring at him? Were you huffing and fuffing as well? Why not just say, when he sat down with it "ooh, can I nick a bit?"
I think he was unreasonable to swear at you.
I also think he didn't do it out of nowhere, and suspect you were giving him looks, or making disapproving sounds
You have put him on a diet, so he'll be sensing you watching every mouthful, so he'll be on the defensive.
Really, I think you created a problem by glaring and expecting an offer, rather than grabbing a fork and saying "budge up sexy, let's share"