What can shorten a dogs lifepsan, yet not kill it?

Im getting really tired of typing the story btu here goes. When my mom died her dog and I were to live with my Grammy and grandpa. So the dog is pretty much hers. Over the years she spoiled it and neglected disciplining it and heres the dog from a happy, healthy, energetic trained dog to an old, senile, wild dog. The dog is 14 and constantly barks. nonstop. and it will not shut up. it is downright VICIOUS to guests and it made my friend one time bleed. he wasn't doing anything, he started petting it and it bit his hand. the monster also nearly bit my grandpas leg off too. he still has a nasty scar. it doesn't know any tricks, it used to know a few but over he years it forgot because stupid gramma didn't reinforce them. It doent respond to its name either. it eats its poo. it wont even let anyone pet it or it barks and jerks away. except for my grandma that is. She thinks it is soooo great but i believe it would be better put down. It has arthritis in almost every area of the body. I can understand why it doesn't do tricks, but the rest is ridiculous. So how can i shorten its lifespan in a painless or almost painless way? not kill it, but subtract a year or two. look, i know this is really cruel and mean but this dog is driving me and everyone but my gramma nuts. Like i said, we cant give it away or put it down. Oh yes one last thing. Frankly, I don't give a CRAP about your comments. The only thing im looking for is answers to the question. If you want to write a comment, fine go ahead, it's not gonna bother me and therefore you are wasting your time. if you still right a comments i will also know you cant listen and cant read. therefore, you should go back to school imbeciles.