The Incredible Melting Man

During a space flight to Saturn, three astronauts are exposed to a blast of radiation which kills two of them and seriously injures the third, Colonel Steve West (Rebar). He is next shown unconscious in a hospital back on Earth, with bandages covering his face; his physician, Dr. Loring (Lisle Wilson), cannot explain what is happening to West or how he survived the blast. After the doctor leaves, West awakens and is horrified to find the flesh on his face and hands melting away. Hysterical, he attacks and kills a nurse (Bonnie Inch), then escapes the hospital in a panic. Loring and Dr. Theodore "Ted" Nelson (DeBenning), a scientist and friend of West, discover that the nurse's corpse is emitting feeble radiation, and realize West's body has become radioactive. Nelson believes West has gone insane, and concludes he must consume human flesh in order to slow the melting. Nelson calls General Michael Perry (Healey), a United States Air Force officer familiar with West's accident, and the general agrees to help Nelson find him.
West attacks and kills a fisherman in a wood, then encounters and frightens a little girl (Julie Drazen) there, but she escapes unharmed. Nelson tracks West by following his radiation trail with a geiger counter, but only finds his detached ear stuck to a tree branch. Perry arrives by plane, and is picked up by Nelson; shortly thereafter, they visit the crime scene where the fisherman's body was found. Sheriff Neil Blake (Michael Alldredge) suspects that Nelson knows something, but Nelson tells the sheriff nothing because Perry had earlier informed him that any information about West was classified. Later that night, Nelson returns home to his pregnant wife Judy(Ann Sweeny), who tells him that her elderly mother Helen (Dorothy Love) and Helen's boyfriend Harold (Edwin Max) are coming over for dinner. On their way, however, Helen and Harold are attacked by West in their car, and he kills them both.
When Blake finds the bodies, he calls Nelson, who comes out to identify them. After Blake angrily demands an explanation, Nelson reluctantly reveals West's condition. Nelson believes West is somehow getting stronger the more his body decomposes. Back at Nelson's house, West attacks and kills Perry, although Judy is not harmed. Nelson and Blake arrive just as West escapes. West then stumbles upon the home of a married couple (played by Jonathan Demme and Janus Blythe). West kills the man and attacks his wife, but she drives him away after chopping his arm off with a kitchen knife. Blake receives a call about the attack and takes Nelson with him to investigate. They follow West to a giant power plant, and then up several flights of outside stairways.
Blake tries to shoot West with a shotgun, but the blasts do not stop West, who throws the sheriff over the railing into power lines, killing him. West hits Nelson and knocks him over the railing, leaving the doctor hanging on the side. Nelson appeals to West, reminding him that they were friends, and West decides to pull Nelson to safety. Two armed security guards then arrive and, in a panic, fatally shoot Nelson as he tries to protect West. An infuriated West kills the security guards and stumbles away. After collapsing against the side of a building, he slowly melts completely away. The next morning, a janitor finds his gory remains and casually mops them into a garbage can. The film ends with a radio news report about another astronaut team being sent to Saturn.

Question: Who kills Nelson?
Answer: Security guards