Klaus wasn’t a good person — his words, because of course he would dare kill everyone that talked behind his back. No, he wasn’t good; he was a monster, an abomination or whatever his supposed father had called him.
A curse had been placed upon him and he wanted it gone — was it really a bad thing? Did that made him the monster of the story? Probably, especially when he tried (and succeeded) to kill everyone that put themselves on his route.
Yes, he was the monster that parents taught their children about, warning them of him. He was the big bad wolf, the croque-mitaine; so many stories had been made about him and his killing.
So why? Why did he wanted to protect you? A relative of the Gilbert’s, passing by Mystic Fall for a quick hello the same day he decided to sacrifice Elena. He could use you — he could.
From the moment his eyes had met yours at the Mystic Grill, Klaus knew that he couldn’t let you die. He couldn’t let anything happen to you — to his soulmate. The one he had waited for, for so so so long.
And the worst thing was probably your lack of knowledge about the supernatural world.
The fact that you didn’t know what was going on in this awful town or what type of monster you were currently having a drink with.
“So, love, tell me more about you? What are you doing in this forsaken town?” he asked you, ready to listen to your voice for hours and hours on end, going as far as forgetting about the full-moon and the sacrifice.