People underestimate Beatrix. Do never forget that Pai Mei chose her to be part of the White Lotus Clan, he even taught her his most secret technique, as the ultimate honour. That means she had reached, in Pai Mei's view, a level of mastership that far exceeded that of Bill, since even Bill was never taught the secret, and believed nobody could hope to be taught. I take that as a proof a her capacity to beat them all fairly, which she did. It is possible though, as you mentionned, that Bill wanted to make sure of his defeat, because fundamentally he always loved Beatrix, and he even tells her at some point that he had dearly regretted "killing" her the first time. He could not do it a second time, that is for sure. I think he just wanted to go more softly.