Klaus gets a kid and Katherine gets to be human again. Sure, the latter was a defense on Elena's part, a way of saving herself and stopping Katherine using what she had on her. Older and stronger, Katherine was no match for Elena — but as a human, she no longer posed a threat to her vampiric doppelgänger. He understood why Elena did it, and likely would've done the same had he been in her position, but that didn't mean he was thrilled about Katherine Pierce being human again.

That, combined with Klaus having a child on the way, had to be like a slap in the face for her.

"It's not the kid's fault his father's a dick."