Aww, I bet the only reason why Bill wanted to kill off Mabel was that she was annoying and because he loved dipper and because he also didn't want to be gotten rid of.
No, killing Mabel would cause maximum pain to the surviving Pines. If Bill loved anyone (which he doesn't. Except maybe Liam. See PengyChan's "Flat Dreams".) it would be Ford. I simply don't understand BillDip.