This kindly old man might give you a case of the warm fuzzies—seriously, he even made George from Seinfeld weep while watching Home Alone by himself.
At first, Buzz tells Kevin that Old Man Marley—a neighbor who shovels snow and salts the neighborhood's sidewalks—is actually a serial killer who makes the people he kills into mummies by placing them in his trash can full of salt. This is obviously not true, but Kevin believes it and runs away every time he sees Marley.
First, he screams after going outside and bumping into Marley: he darts back into his house. Then, at the drugstore, he flees when Marley enters, unintentionally stealing a toothbrush in the process.
Even though we could easily assume Marley isn't a serial killer—nothing Buzz says is trustworthy—we see him as Kevin sees him. Marley's eyes look intense and his boots clomping on the drugstore floor look distinctly serial-killer-ish.
But then we learn better.
When Kevin goes to church, Marley sits next to him and says,
MARLEY: You live down the street from me, right? You know, anytime you see me you can say hello, you don't have to be afraid. A lot of stuff has been said about me, none of it's true.
This immediately diffuses all tension, and Kevin isn't afraid of him anymore.
However, Marley himself is still afraid—"You're never too old to be afraid," he tells Kevin. He can't celebrate Christmas with his son because of an argument they had years ago, and can't spend time with his granddaughter. Kevin urges Marley to make up with his son, telling him about how he overcame his own fear of the basement.
Kevin hasn't appreciated his own family (and they haven't been very nice to him) but they're still on speaking terms. Marley's sad case demonstrates what can happen if the lack of appreciation gets way out of control. You can end up losing something really valuable purely through being stubborn and insensitive.
Marley's a cautionary tale for Kevin. Even though he's a really nice guy, Kevin doesn't want to make the same mistakes as him.
Later, Marley saves Kevin when the burglars finally capture him—he sneaks up from behind and bashes them over the head with his snow-shovel, allowing them to be arrested. (Anton Chekov said that if there's a gun in the first scene, it will be fired in the last—the same is true with a snow-shovel and smashing people on the head).
The next day, Kevin looks out the window and sees Marley making up with his family members and hugging his granddaughter. Aww—he knows that he's given something to the old man too, helping him find the courage he needed. They exchange a friendly wave.
And then Marley whips out his snow-shovel and murders his whole family. What a twist. (We're lying. But maybe in a spinoff?)