I am so sorry to hear of your situation. There is absolutely no question about what you must do - you must leave. The probability of something happening is based upon the times it has happened before - if your husband has threatened to do things before and has carried them through, then the threat he has just given you should be taken very seriously. The fact that he has treated you for the past 19 years in the way that you describe is intolerable and unacceptable. No doubt he has ground you down little by little, so that you are somewhat unable to see how serious this is, but I would consider your husband's risk level to be high.
Well done for reaching a point where you have had enough and for wanting a divorce. You are aware that your husband will find this unacceptable and could be the trigger that he needs to take his behaviour one step further. Try not to inform him of your decisions - keep it very low key and make plans to leave. Use someone else's computer if you need to to find a women's refuge, possibly in an obscure area where he won't trace you. You will need support with this and they will believe everything that you tell them. They should help and advise you with regards XXXXX XXXXX police and with staying safe. I would suggest that moving to family or friends better than staying at home, but would be more obvious a place to find you, and a source of more emotionally complicated, less professional support (as much as they may love you).
My advice would be to not hesitate in your actions. You are aware of the possible consequences and it is important that you act upon what your conscious and your subconscious parts of your mind are telling you - you are at risk, get yourself away as soon as possible.
With Best Wishes, Sarah