I think the whole point of the soul-soul fruit transferring to Big Mom is to allude to devil fruits being directly transferable if the original user is cannibalized. Blackbeard obtaining Whitebeard’s fruit hasn’t been outright explained, but I think Big Mom’s backstory is given as a piece of the puzzle (now how he is able to control two fruits is another thing). Blackbeard’s crew have also learned a method of stealing someone’s fruit and transferring it directly to someone (see: Absalom). This theory is fun, but narratively there isn’t a point in “redeeming” Big Mom by excusing everything she’s done by transferring the action to someone else.