Dilemma

So Rene and I have this dilemma..... our wonderful dog, Ash, has some issues. Now, he is a rescue dog and we have no idea what his past was like but we do know he was covered in fleas when rescued and had no fur on his hind side/tail. Since then there have been some other things which have occurred. About a month after we had him he bit me, he broke the skin and it wasn't too major. However, this happened totally unprovoked. I was lifting him up just as I had done for the past month, but this time something just snapped. Since then he's bit the vet (which was done upon fear and the vet said not to worry) but he's also bit Rene when Rene didn't give Ash his treat quick enough. And then tonight I was playing with him as I've done forever... Rene had just finished playing with him and then Ash came over to me. We were just playing and then SNAP, he bit me. This time he got me on the arm and there are 4-5 puncture points. I lifted my arm while Ash was "attached" and so the puncture wounds dragged down my arm. Now, the dilemma --- what do we do with Ash now?? We don't/can't trust him. We can't have him around anyone else. We love him.... But what should we do? Is the best thing to put him down? We've tried to love him, we've taken him to training and nothing has worked so far. It just hurts me, I wonder what Ash must have gone through with his previous owners that made him this way. It's not his fault, but now he has to be punished.... It breaks my heart.