I can only answer the bottom questions, so ill try.
The Thing is, revenge isnt so cool. Lets say B.B. would have died, and the Bride would have killed Bill without getting her daughter. Her new friends are dead because of Bill, she killed her old friends, she would be totally alone now.
And to ur last question, thats the thing with revenge, ure just as bad as the guy you want to punish. And imagine the crying of all ZOMGACTION fans if she wouldnt have killed Bill