Why does my 3 yr old throw violent fits??

[babym2]

So my son is 3 yrs old i also have a 19 month old little girl also. My son was the easiest baby EVER and all the way up untill he was about 2 was Great no problems what so ever.  However he now is absouletly scary! I lay him down and he will throw a fit untill he falls asleep and then first thing in the morning he starts! I can not go to a restraunt with him, he will cry, kick the chair, and be extremely rude. He will hit his sister and push her down for no reason what so ever. If you ask him to do something he will stick his tounge out at you and call you stupid, or scream words that he made up! He hits the dogs, will kick at me and his father! He refuses to take a nap now so we moved to just letting him lay down but either way he will get in to something. He will pee and poop on the floor when hes angry! Several times he pooped in a toy and wiped it on his sister, i have to check on him every couple min other wise he gets into something! I have gotten gates,locks for the doors and he still manages to get thru them! PLs PLS i am lost i feel like i am loosing my son, I have a brother who was a foster child so i let him move in with me come to find out he molested a 6 yr old little girl needless to say he is NOT around my kids now however he had plenty of alone time with them. Could this be a sign of molestation is there something that im doing wrong? PLS let me know!