Good evening, I would like to work to assist you today.
I can most certainly understand your worry and concern at the moment. My first suggestion would be that you take your child to his pediatrician, and mention this behavior to him or her. In this way, the pediatrician can examine your son to diagnose any possible health concerns that may be prompting your son's aggressive behavior, and he or she should also be able to recommend a goo child therapist that specializes in helping children and their families learn control aggressive thoughts and behavior. I also encourage you to continue to have conversations with your son (and include his grandparents as well) on the proper ways to express his anger, and explain to him that killing someone is forever, and that there is no coming back from death, explain to him that you and his grandparents love him very much, and would never think of hurting him in any way, and want for him to love and never hurt you all either. Suggest to his grandparents that they explain to him how they were very hurt when he threatened to kill them, and grabbed the knives. They should tell him that all they want is for him to be happy, and that is why they spend time with him, take him to the movies, etc. Let him know that if he ever feels unhappy or angry, that rather than hurting or being mean to someone, he should come to you and talk about his anger so that you can help him get rid of the anger and be happy again.
Also, be aware of the things that may be influencing him to think and act in such aggressive ways; television, video games, friends or other outside influences. Be sure to monitor your son's daily interactions and activities so that you can pin point where he is learning this aggression from. Also be aware of your own emotions, and the things that you say or do around him, as children his age are very impressionable, and are developing mentally and emotionally at a rapid rate. Continue to encourage his positive behavior and reward him for that; and within reason, time-outs, helping you with chores, taking away special privileges until behavior improves. Also, employing reading about controlling anger may be good for him. Here is one book, "Angry Octopus" and "When I Feel Angry" that may serve as tools to help your son with his aggression: http://www.amazon.com/Angry-Octopus-Relaxation-Lori-Lite/dp/0978778170/ref=pd_sim_b_23
Most importantly, do your best to remain calm, but dedicated to helping your son at this time. Along with seeing your son's pediatrician, and any therapist suggestions that he or she may have, Goodtherapy.org allows you to search by zip code and do an advanced search for a therapist that best suites your family's needs: http://www.goodtherapy.org/; Do give this website a try if nothing else is working. If you need any different ideas or insight, or if you have any questions or comments about my answer, please message for me, "earthsister", and I will be more than happy continue assisting you.
And pardon the typo, that is: "more than happy to continue assisting you." Thanks.