Buffy was more prepared to listen to Dawn than Ben. Dawn was her sister, not just a disguise for a dimensional key. She was blood, she was family, and in Buffy's mind, that was never not the case. Ben might've been innocent. He might've wanted nothing more than to just walk away from this and do whatever he could to keep Glory at bay. But he hadn't had any luck so far, and Buffy wasn't going to just sit on her hands and let him go, let them go, when she had the perfect opportunity to end it.
"Ben," she said warningly, giving him a pleading look of her own. She didn't want to hear it. She remembered Ben back from when they first met, remembered the feelings and emotions and the time they'd spent together. But it was over. It probably hadn't even been real.
"I'm sorry."
And then, without hesitating, she brought the sword down and plunged it right into Ben's chest.