We put two females down because they had 2 terrible fights. We adopted a Rott who is starting to growl when we rub his belly. We do not want to put him down too. What can we do?

I just read your article about breaking up a dog fight. I loved it and thought the information was wonderful. I wish I had read it 6 mo. ago. We had 2 sister females, had 2 terrible, terrible fights and finally had to put them both down for being too aggressive. The fight wouldn't have lasted nor been as traumatic to the family had we had your information. Aside from this, I'm hoping that although this has nothing to do with your web page, praying that maybe you can help. I cant find any information any where and unsure what to do except put our current dog down, who is a male Rottweiler, about 2-4 yrs old, (this was a stray that came and stayed) he was very, very sweet... obedient and loving for about the 5-6 months that we've had him. Last week he began growling under his breath whenever we pet him, and a few days ago has also started baring his teeth. He is an outside pet and as was my normal routine, I would go out several times a day to spend time with him, playing, rubbing his tummy which he loved, etc. He continues to approach me for his tummy rub, but when I do, he begins the growling, etc. What is happening to him. He has also started chasing cars which he never did before. Do we have any choice but to put him down? And why has he started acting like this out of the blue? I'm so sorry to impose on you like this, but you seem to be more informed than anywhere else I've read. Thanks so much.