I am so so sorry. I do know the fear of your own child. My oldest is now a great person but was very very violent as a child/young teen. I was pretty sure he was going to end up in jail for hurting/killing someone and am thrilled that this is not the path he has chosen.
I do NOT think that your son can or will change. My son did NOT have the problems yours does, and did not have the history of animal abuse. I am sure your son got a bad hand genetically from his birth parents and he was abused by them. This is NOT your fault and at this point not something you can hlep him with.
As you have other children, you need to INSIST that the case worker find an IMMEDIATE out of home placement for him. do NOT agree to take him home with you, regardless of what they say. He is a clear danger to others, hence the threats to kill/rape and as a RESPONSIBLE PARENT you cannot expose your other children to this OR allow him to be in a situation where this is possible. Therefore the state must place him somewhere so that he cannot hurt anyone.
IF you cannot get this done TODAY, make SURE your daughter has both pepper spray AND an alarm with her at ALL TIMES, ESPECIALLY AT HOME. The other kids may be too young to handle pepper spray with-o getting hurt, but they can also have personal alarms that will bring you running. THey are not terribly expensive and you can find them at radio shack, walmart, etc... Check the sporting goods and the home improvement areas. Also check where they sell keychains. Contact the court to have your daughter and other children file for restraining and/or protective orders against your violent child. It will likely force the courts to put him in a placement out of the home, or force DHS to do it.
You may have to have your other children sleep in a room with you and/or your husband. Maybe the other boys with husband in one room and you with the girls in another, with your difficult child in his own area.
ALL of the family NEEDS to be in intense therapy NOW. Once the kids are safe and are positive that this boy will NOT come back, you are going to learn about a LOT LOT LOT of TERRIBLE things he has done. To, with, for the other kids. It would be very very rare if he has not already abused them in some ways. They likely have not told you because he has threatened them and they are sure that he WILL follow through (which he likely will - do NOT think he won't do what he says. You cannot afford to do that.)
I am so sorry to sound so negative. Your son most likely has Reactive Attachment Disorder (Reactive Attachment Disorder (RAD)). If you haven't heard of it, there are a lot of good books on the subject. There is treatment, but improvement is incredibly hard to bring about. With other kids in the home, having your difficult child lvie with you just isn't safe. Make sure the other kids know that they will NOT get into trouble if they have to stop him from hurting them, and that he has NO RIGHT to abuse them.