Hung Out to Dry
The song "Lean on Me" isn't in this movie—maybe they couldn't afford the rights to it?—but it would be a good song for the scene in which Karl helps Frank carry sacks of laundry. Here are the lyrics we like for this scene:
If there is a load you have to bear
That you can't carry
I'm right up the road
I'll share your load
These lines sum up Karl and Frank's friendship in a nutshell. Karl helps Frank bear what he can't on his own. We're not saying that he can't-can't, because Frank has been carrying the laundry for a while. But Karl makes the trip easier.
The same goes for Frank and Doyle. Frank is strong enough to handle Doyle's abuse, but Karl believes that Frank shouldn't have to do that. Karl takes the pressure off Frank by standing between him and Doyle.
Karl's helpful nature also plays out in his profession: he's a fix-it man. Symbolism doesn't get more in-your-face obvious than that.
Karl's desire to fix things underlies most of what he does throughout the film. He thinks that by murdering Doyle, for example—a task he accomplishes with a simple tool, as if he were just repairing a small engine—he will "fix" Frank's life. And before killing Doyle, Karl fixes Linda's washing machine so that Frank won't have to carry laundry again.
Karl sees himself in purely utilitarian terms. He is a man who can wield simple tools, whether those tools are for repair or destruction. He doesn't seem to understand that Frank values him for more than just what he can do. Frank appears to value Karl for who he is, and that's more than a repairman.