my 3 year old wont listen and gets very agressive

my three year old son has been having behavior issues for the last year and a half at first i thought that they may be just typical " terrible twos" and "threatening threes" type behavior. tried to ride it out and get him interested in other things tried discipline. hes dad and i have been separated since he was about 7 or 8 months old and we haven't really gotten along until recently and my fiance and i turned to my ex and his wife for help. the only problem they have with him is pulling hair and biting. we have problems with him hitting his sister (age 4)  with remotes, at grandma and grandpas when he was visiting hit his cousin with a crowbar out of grandpas garage. he is extremely aggressive to animals to the point we had to get rid of our to dogs because if we would turn our backs for a second he would strangle them, kick them, hit them to the point we were worried he would seriously hurt them or even kill the puppy.he throws temper tantrums half the times were out but on the other hand he will surprise us and be a perfect angel. he also gets destructive towards objects he took a stick and broke my gas tank door or will throw toys when he gets upset. not sure what to getting to the point of pulling my hair out help please.  

P.S. the men on his dads side of the family all have agression issues dont want him to turn out the same way but at a young age already seems to be running down the same path.
PLEASE HELP!!!!!

[Kevin Kennedy, Ph.D.]

He is definitely not developing on a goood course. It sounds like the family situation itself is a problem, so in addition to having him evaluated it would be prudent to widen the focus by examining the interactions of the family and how those interactions are having an impact on your son. There is a simple rule of thumb regarding pre-schoolers and behavior: The more the adults change, the more the child will change. So, in addition to focusing on your son, look at your own functioning/parenting and that of the other adults in his life.