Maybe he didn't know her. He'd always imagined he did. His flighty little sister, mean streak, couldn't stick to anything for too long before she got bored of it or it of her. But she had been with him the longest of any of them. She'd stuck by him through a lot and in spite of her betrayals she, and he supposed Elijah had generally stayed their course with him. But there would always be a part of Klaus that didn't know how to be anything other than what he was. Bitter and wary of anyone that promised him anything because one day, eventually, they would turn their back on him. Better that he do it first.
She'd gotten it right with the trade of course. Her blood so that he would test Elena's. But did she really think that low of him. "I was going to offer it yes, then compel him to do what I asked. Your blood would never have been in danger sister but I needed the failsafe." In case of vervain really, but whether he'd have handed it over or not, he didn't actually know. He doubted Holmes would do the deal without compulsion.
He didn't know how to be a better brother to her, or to Elijah, the kind they wanted.
Oh he'd tried, he'd tried to force Katerina out of Elijah's life forever. He'd tried to ensure Rebekah could be happy, but she'd turned away from him so many times, over that Hunter, and over Stefan. He didn't want it to happen here. But without the daggers to keep his siblings in line, he was losing control of them. He didn't like it and in all honesty he didn't know what to do with it.
"What do you want from me? You asked me to promise to ignore the chance for hybrids, said you'd stand by me. But you won't Rebekah, you never do. Always and forever, you swore it to me but do you mean it, did you ever?"