Do you ever feel like you are the WORST Mother in ...

Do you ever feel like you are the WORST Mother in the world? I do. My sweet boy is not so sweet anymore. He used to be sweet and kind and caring. He used to love other kids and would play with whomever was around and had no problems with it at all. He never balked at a child taking something away from him, he was easy going and nice. This is not the case anymore. My child has been taken over by a mean, screaming, toy-stealing, chair-pushing, jumping on little kids, MONSTER. I don't know what I've done so wrong with him. He is so nice to Amelia, for this I am so grateful...but to other kids, not so much.
On Sunday, we went to our new church for the first time. He went into nursery with the other kids (ages 18 mo. to 3 yrs.) and was understandably nervous. So, I stayed with him, no biggie. He stayed on my lap and wouldn't talk to anyone. His teacher came over to introduce herself and he buried his head in my chest, fine. However, when a child took a toy that was on the floor that he was eyeing, he SCREAMED! Not just any scream, but a high pitched, shrill scream. The other parents that were in there looked at me like I told him to do it. At another point he pushed a chair into a little girl, pounced on another (they were playing a game like tigers, but he did it from behind and no one else was pouncing), and hit one of the adults (she wasn't very nice, but STILL!).
Then, today we went to playgroup at the church. He was SO mean to the other kids, again! I just don't know what to do with him. I can't stay in my house all the time for fear of what he'll do to the other kids, but I don't want to take him out and have him potentially hurt other kids.
I know, he's just gone through a big change. He's moved from the only home he's ever known (the home he was literally born in), all his friends are 1 to 1.5 hours away, and things are different now. But, it is breaking my heart watching him being so mean. Any advice???