Right. So when we meet him he is a paranoid loner. This paranoid loner character invites someone into his home and leaves him unattended within MINUTES of this man coming on his property??
The only times Frank was left alone was while showering and briefly in the dining room as Bill walked in with the lunch. I don't know why you're mentioning this.
Sure, we see him evolve over the next 20 years. But as a viewer watching the show they told us 1 thing about him: He's a paranoid loner. If he is going to betray that within the first few minutes he is on screen, why tell us that at all? Why not tell us he's a man who is desperate for human interaction? His wife died and he's been looking for human interaction?
They could have told us any story they wanted. What they did tell us is this character is wildly inconsistent from scene to scene with no reason given. How the writing is being praised in particular is very confusing to me.
As a paranoid loner, what's the lie Bill tells himself? That he doesn't need anyone but himself.
Over the course of the story, this belief is challenged. First, he meets Frank, and a physical relationship turns into a romantic one. This is the first challenge to his belief. It's not all smooth from here though, because Bill and Frank clash over their philosophies, and Frank wants them to meet
even more people.
Eventually this leads to the lunch with Joel and Tess. Bill has a gun trained on Joel throughout the lunch, and tells him that he and Frank don't want or need their help. However, Joel informs him that his fence won't hold and that Joel can get him materials to build a fence which would last a lifetime.
Initially it seems like this is going to be a mistake by Bill and will lead to Frank's death. However, when the couple is attacked years later, the fence holds - revealing he did agree to trade resources with Joel. Another challenge in the belief.
The same night, when he's shot and believing he's about to die, Bill pleads with Frank to call Joel and Tess; that Frank can't be there alone. His previous belief system has all but crumbled.
Bill's final character arc ends years later when Frank is dying and asks for his help with an assisted suicide. Bill also decides to kill himself, deciding that a life alone isn't one worth living. His belief system has come full circle.
That's my long winded explanation of why the writers made Bill a paranoid loner. He's the protagonist of this episode (basically a movie) who's going through change. If his character background is that he's desperate for human interaction, then the writers needed to tell a different story. Adding that detail to this story would've made it less nuanced and frankly generic.
