Everything is Jake Spoon's fault.
When Jake rides into Lonesome Dove, we see it happen through Newt's eyes. Jake Spoon is a hero. Newt's idol. Other people, on the other hand, have a more realistic view of Jake. Gus, for example, thinks that "something in Jake didn't quite stick. Something wasn't quite consistent" (6.32).
So who's right? Well, over the next few hundred pages, we see Newt's and others' idolization unravel as Jake's true nature is revealed. He is consistent: consistently making terrible decisions.
Pretty much everything about this man is dismantled in front of us. It seems his whole reputation as a sharpshooter was based on "one lucky shot" (6.122) in Mexico. Lonesome Dove, by contrast, begins with an unlucky shot. Jake kills a dentist by accident in Arkansas, and now he's on the run. It's his idea to go to Montana, a beautiful place he saw two years ago.
But the guy's got no follow-through.
"Jake just dreamed his way through life and somehow got by with it." (6.62)
He hitches his wagon to Lorena, a woman with more ambition in her little finger than he has in his whole body. From the beginning, he knows "it wasn't meant to really happen" (14.17). Yet he neglects to tell Lorena that. What a cad.
"He'll always find somebody to take care of him" (17.48), we're told—but Lorena isn't looking to be a caretaker. Not of a grown man, anyway.
At first, you might feel sorry for Jake Spoon. He's lost. He's in over his head. But he quickly becomes angry and violent toward Lorena. He hits her. He tells her, "[O]nce a whore, always a whore" (40.14). And he rapidly spirals even further down from there. For example, he doesn't pursue Lorena when Blue Duck kidnaps her; instead, he falls in with a band of horrible criminals.
When Gus and Call catch the criminals, they are sort of surprised to find Jake with them. Sort of. It isn't like Jake to commit heinous acts like hanging innocent people and setting their bodies on fire, after all. But at the same time, Gus and Call are not surprised, because Jake Spoon is a weak man who goes with the flow, even if that flow is a river straight to hell.
You might still feel sorry for Jake Spoon as he's about to be hanged. But then Gus tells him Lorena is okay. "Who?" Jake asks (72.158). Man.
At that very moment, it's hard not to want to hang Jake yourself. But Jake is kind of the ultimate tragic figure in Lonesome Dove, whether you feel any sympathy for him or not. He has an eye for beauty, beautiful women, and beautiful country. When the cowboys see Montana, they realize, "It was plain that Jake Spoon, who had been wrong about most things, had been right about Montana" (93.4). Jake knows beauty, but he doesn't know what to do with it.