Stephen King's The Mist Wiki has a more comprehensive article on the same subject. For it, see http://stephenkingsthemist.wikia.com/wiki/Ambrose_Cornell
Ambrose Cornell was a character in The Mist. He was trapped in the convenience store along with many others. He sided with David Drayton. In the book, he fled back to the market, but in the film, he was eaten in the mist.
A man in his late seventies, Cornell joined David's side and disliked Mrs. Carmody. He also claimed that he had a shotgun in his car and when a biker offered to get it, they tied a rope to him so they could keep track of the biker. The rope went taut, then became bloody as they reeled it in, and all that was left of the biker was his lower torso.
After Ollie Weeks had successfully killed Mrs. Carmody, Cornell, Myron and Bud Brown fled along with David's group through the parking lot. Myron was devoured by a spider, Bud fled back to the store. Cornell, lost in the thick clouds of the infernal mist, was then surrounded by the spider-like creatures and devoured in the midst of some cars.
Behind the Scenes
Cornell was portrayed by actor/painter Buck Taylor.
Also, when Cornell dies, the last scream heard is in fact the infamous "Wilhelm scream" - a famous cinematic phenomenon which has been used in over countless movies and television shows.