So in the scene the dog was super close to Will and they loved each other. They had the dog before the apocalypse and when his wife and son was evacuating via helicopter. Before they left the son gave Will the dog and says "Take care of doggy!" and leaves. But before they leave another helicopter gets attacked by zombies and it crashes into the one Will's family is in. So the dog and Will have been close to each other ever since. Years later, (I should probably mention that the zombies are very intelligent) the zombies set a trap and traps Will and then they get attacked by zombie dogs. A dog almost kills Will but his dog saves him and takes the hit so his dog is now infected with the virus. So they leave and go to the lab and Will sees that the virus is making his dog to aggressive so Will sits there having his last moments with his dog. So when his dog becomes 100% infected Will has no choice but to suffocate the dog by strangling it. Go***amn i cried while typing this... ;-;
