We are struggling with the same thing with our lab/mix that we got as a puppy from rescue. Whether it's from bad breeding or poor socialization, he has a very unpredictable side to him and has bitten 3 times. We were very lucky the first time...he bit our son's friend in the face. He is very dog aggressive. I was walking him last week and he lunged at someone.
Most of the time, he is fine. But like my vet said...This dog will be fine 95% of the time. It's the 5% that you have to worry about, because you never know when it will happen.
I want to put him to sleep, but it's the 95% of the time that my husband and son are attached to.