Ok, this is a long story, but we are at our wit's end. We really need objective advice.
Two years ago we adopted Charlotte, a "Schipperke" mix (I quote Schipperke because frankly we have no idea what she is; she weighs 30 lbs, has the face of a skippee, looks a little like a small black lab, has a spotted tongue like a chow.) She was two and a half at the time, we adopted her from a no-kill shelter, and we had no history on her.
Once we got her home, she exhibited fear-growling tendencies with most of our guests pretty much from the start, although she was kind to us when we met her at the Petsmart adoption clinic.
Then one day we were having a dinner party. We had guests sitting on the floor, on sofas and on chairs around a low coffee table. We were all eating. Charli was 

laying with a woman who she had met several times, and with who she was very friendly. Suddenly Charli pinned her down and nipped at her face, drawing blood from her lower lip. Needless to say we were scared - this was the first violence we had seen from her, especially so since she had met this lady on a few occasions before, and was never afraid of her.
Some time later, we had a few guests over, and we were watching a movie. Charli was sleeping belly-up in a friend's lap for two hours, totally content and comfortable. She had met this guy before also, and was soundly sleeping in his lap. For no reason whatsoever, she sat up, waited for a moment, turned to him, and snapped at him, drawing blood from his face near to his eye. This was now the second time she had snapped at someone whom she knew and "trusted."
Last night, a friend of mine was over, again someone with whom Charli was friendly. She had been playing fetch and tug with him for several hours, without incident. She began to nap and we set out to study. After some time, he got up to go to the bathroom, and walked up to Charli on the way, just to pet her. He had done this several times before this evening, but this time she snapped at him and nipped his chin, drawing blood with a small puncture wound.
Does she need to be euthanized? Is sedation an option? We are scared and we love her and we need help. Can anyone give us advice? There are people with whom she behaves very lovingly; members of our families and some very close friends. We have hired a trainer, tried leashing in the house, tried muzzling, crating her when anyone comes in the house; we are unsure what to do, but we know that something must be done.