This might seem a bit far-fetched at first, but truthfully, this theory has some intriguing evidence behind it. On a different Reddit thread, u/jordanlund presents a full case complete with screenshots, and it’s honestly quite convincing.
Here’s the gist: while Beatrix does, in fact, set out to kill Bill, the bloody trail that leads her towards her former beau ends up making her feel guilty. After killing O-Ren Ishii (Lucy Liu) in the first volume — if you twist the on-screen events into chronological order — she then travels from Japan to California to take out another former Deadly Vipers assassin Vernita Green (Vivica A. Fox). When Vernita’s daughter Nikki (Ambrosia Kelley) discovers the bloody aftermath, Beatrix is uncharacteristically bashful, even apologizing to the young girl and offering her a rematch one day.
So, as the theory goes, Beatrix doesn’t directly murder anyone from that point on. Michael Madsen’s Budd gets bitten by a poisonous snake at the hands of Daryl Hannah’s Elle Driver, and Beatrix elects to remove Elle’s one remaining eye instead of killing her. (It’s reasonable to assume that she either succumbs to that injury or is bitten by the still-present snake, though.) Not killing Bill at the end would certainly fit within this theory, and there’s more!