Need Guidance for Delicate Situation in NYC

Hello ISYN. I am a 24 year old English Nanny working for my third nanny family in four years. I am new to this position. Something has come up that has never come up before and I have addressed it with the parents, but I am not satisfied with the resolution. I take care of two school aged boys, close in age, 7 & 9. The younger of the two is, for lack of better words, physically violent. When he does not get his way, he will kick his brother, hit his brother, throw things, punch the wall, etc. He has not ever hit me as yet. The older brother is very shy and stays away from his younger brother as much as possible. This alone, is sad to me. When I mentioned specific situations to the parents, they said, "that is how X is, he'll grow out of it" and "I hope you let him know how unacceptable that behavior was". I have been here less than six weeks and I think that this might be why the parents have went through a quick secession of nannies. I am also concerned that the child could hurt his brother, himself or another child while in my care and I could be held responsible.  We've yet to go to the park or anywhere outside of the home, walk to school but I am not looking forward to this. The boys has never hit me but he has told me things such as, "I will kill you" and "cut your head off and bury it." His behavior is generally very normal, except when it comes time for me to limit set, end play, homework time, or the like. I am not comfortable with this for a number of reasons. Has anyone experiences anything such as this?  (edited JP)