The loner is, in many ways, the opposite of the leader. He has no interest in protecting the group or saving humanity. His priority is always numero uno. He probably has a military or survivalist background, is heavily armed, and either has a stash of food and supplies or is able to acquire them easily. In the movies, the loner is a leader in disguise, the gruff fighter with a heart of gold just waiting to find the reason to step forward. In reality, the loner may occasionally hook up with others, but only when it is beneficial to him. He will also drop them at the first opportunity, preferably when the zombies need something to distract them.
The problem with the loner is he has no one to rely on when things go wrong. The simple fact of the matter is that it’s hard to survive in a zombie apocalypse. You gotta sleep. Who’s going to keep watch? What if you injure yourself? What if there are more zombies than one gun can possible hold off? No, the loner might live longer than the leader, but at some point, he’s gonna slip up. And when he does, that’s the end.