When do I take my stepfather's threats seriously? He is dying and told my Mom "I am not leaving this world alone."

My stepfather is a mean and hateful person who has stopped trying to hide it now that he is dying. When do words become more than idle threats? He has told mom a couple of times that he is not "leaving this world without her". He is an avid gun collector but the guns are in the downstairs and until Friday - no one thought he could get downstairs. When hospice care came - he asked them to help him downstairs to look for a DVR book.
Yesterday - he was EXTREMELY angry because my husband and I helped mom with yard work and Xmas decorations. He spent the entire day ranting and raving. His daughter came to visit and he did not stop. He gets so angry that his eyes bulge, he turns white, and shakes. He thumps his cane and you can see the hatred pouring out of him while he restrains his urge to hit with the cane.
Today (Sunday) he was calm and quiet. Mom worked in the other room on cleaning kitchen cupboards. She heard a funny noise - peeked around the corner - and thinks she saw him putting bullets in an ammo cartridge. She cant search his area because he never leaves it. She went downstairs and saw an empty peg on the board that holds the guns. BUT she isn't 100% sure that there was a gun there.
He has always been jealous of the relationship my mom has with her children and grandchildren. We do not visit them. She comes to our houses. There has been no family gatherings for 20 years because of him. Mom should have left years ago but she is the type of person that pretends things get better. She lives much of her life in denial.
MY QUESTION - WHO do I contact to get help? I know I can call the police for a welfare check - but what are they going to do with a dying old man in rural Iowa? Nothing - he can't go to jail here. Courts take too much time and if he has any idea something is going on - it will be too late. His daughter is the hospice contact. She is totally intimidated by him. She is always trying to placate him and tells my mom "he's dying - it's all talk." The rest of the siblings do not visit or talk to him. They dislike him because they put up with him all those years before my mom came along. When he had her to bully - he left them alone. They could care less what happens to him.
YOU ASK - why doesn't she just walk out? Because she bought that house before they were married and it is her sanctuary. (OR was until this became too much to handle). And when he is gone - it will be her sanctuary. She is 74 years old and still works everyday because she enjoys cleaning other people's houses. She has filled her house with pretty things that he is threatening to destroy if she follows through with her threat to walk out.
She is safe tonight because she is in her bedroom with the door locked and her granddaughter is sitting in a car on the street watching him sit in front of the television.
I need to talk to someone in the morning but do not know where to go? Police? Lawyer? Doctor? He has COPD, untreated diabetes, pacemaker, defibrillator, large tumor lung cancer and I don't know what else.....He is not on any pain med except Tylenol.....takes a diuretic, stool softener, and a couple heart meds - nothing else for mental or emotional diseases. I find this incomprehensible.
Sorry so long - and thank you for any insight you may have. I am hoping that someone else has dealt with this type of situation.