A difficult decision

I've decided to have Taboo put down. I've been thinking about it for awhile. He's old and sick more often and has fat tumors all over him. But the main reason is that he keeps biting other peoples dogs. Yesterday, he bit our neighbors dog. I was standing there talking to the neighbor and the dog were sniffing each other and out of nowhere, Taboo just bit her right on the snout. I checked the dogs face for any cuts and didnt see anything, but about a half hour later, my neighbor called saying that she found a cut right under the dogs eye. Last year, when we tried that roommate thing for awhile, Taboo bit her dog, and the year before that when we tried to get Avery a puppy, Taboo bit him too.

He just keeps doing it and his behavior just keeps getting more aggressive toward people too. He keeps barking ferociously at the other neighbors when they walk past. I am very afraid that the day will come when he will bite a person, so I'm just going to have him put down. I dont know what else to do. I dont want to have a dog that I cant even trust to put out on his chain. I just cant risk it. It makes me so extremely sad to think about not having him anymore. He's been with me for so long and is such a solid part of my life. I've had him since my senior year of high school...almost 12 years now. Through all of the ups and downs in my life, he has always been there. He's always been my rock. Without him, I will truly be all alone.