Add the bacon pieces to a large saute pan and cook them over medium heat until the bacon is lightly crispy. Place the bacon on to some paper towels, and blot carefully to soak up the extra grease. Add the beans, bacon, onion, bbq sauce, brown sugar, ketchup, molasses, and mustard to the slow cooker and stir it all together. Cook the mixture in the slow cooker, and serve hot.