When Buffy lifted the sword above her head ready to end him ready to kill him Ben thought about maybe lunging at her, but he figured she'd pull him down with her and that would defeat the purpose entirely. So he relied on the on thing he had left that might work. And it was even true.
"I was falling in love with you...back in Sunnydale...here... Buffy I love you..."
It was a cheap shot of which the Hell Bitch would be proud. But it was all he had left to try.