"parenting" concerned parent?? help me?

Ok, I am a little concerned about my 3 1/2 year old son. He seems to me, to be a little mean. Im not sure if its normal behavior or what. The most recent, and was very concerning to me, was he through our 3 month old puppy down the stairs. She is completly fine suprisingly, but I am concerned for him. He threw her behind the couch. He is mean to his brother, he will just push him down. He hits our older dog in the face. He says mean things to me, like "i dont love you any more" or "i dont like you" we dont say those things, he says "i hate you" but at the same time he can be vey sweet and he will ask to cuddle and he is all boy. but sometimes he just relly concerns me. I just dont know or think its normal! Does anyone else know? or have any advice? i know harming animals is something to be concerned about, i dont want him to grow up and be like a serial killer!! help me!!