I first learned about this fan theory from one of those top 10 videos on the Screen Rant YouTube channel. It's a rather simple premise based on a small segment of a movie but it does make sense from a figurative viewing of the scene. In the Home Alone movie Kate McCallister (Catherine O'Hara) may have unwittingly sold her soul. The late John Candy was credited as the character Gus Polinski but the closing credits of the movie might have just as easily listed him as the dark lord Lucifer himself.
I wrote a post about outsmarting a demon back in 2018. That doesn't help the Kate character but it might save you someday. Be sure to check that out after reading this post.
As the YouTube video I linked to in the first paragraph points out the airport seven miles from Scranton, Pennsylvania is near a crossroad. That itself may be coincidence because other airports exist that share that aspect but the point is that Kate McCallister was near a crossroad in the scene.
Image Source: Google Maps
Wikipedia has an entry for the crossroads mythology. It doesn't always have to be the Devil because some stories just feature deals made with demonic entities. The deals tend to be more powerful when made with the Devil though and Kate McCallister virtually invokes the Devil specifically.
Kate's words would have presumably sounded like sweet polka music to the Devil. An angry tone of pure desperation. The vain and selfish vow she was willing to sell everything she owns with no regard to her family. Kate McCallister was in dire straits and willing to make a deal.
A man identifying himself as Gus Polinski claimed that the group he was with had a "crisis" but seems to already have the situation under control. Perhaps "Gus" was trying to ingratiate himself with Kate. Then he claimed to be the "Polka King of the Midwest". "Gus" proceeded to list his bona fides. It was 1990 so there would have been no way for Kate to have quickly checked the validity of the claims. Even if it was taking place in the current era and Kate could check the internet on a smartphone the Devil could have assumed the physical likeness of a polka musician he desired to impersonate.
If "Gus" was the Devil most if not all of his credentials as a musical artist were lies to just establish himself as an authority figure. The psychologist Dr. Stanley Milgram conducted experiments during the 1960s studying obedience to authority. In one experiment it was demonstrated that an actor wearing a lab coat was enough to influence test subjects in the experiment to obey the perceived authority. A fact that the Devil would both be aware of and a master at exploiting.
As the Screen Rant YouTube video pointed out making a deal with the Devil often leads to an ironic twist of fate. At the end of Home Alone Kevin's mom arrives home just as the rest of the family was arriving. If Kate McCallister had sold her soul for a ride to Chicago it was only for a few moments of being early in exchange for an eternity of who knows what.
One final thing: "Gus" claimed he was the "Polka King of the Midwest". Christmas is the time of year featured in Home Alone. Although most historians would place the birth of Jesus at sometime around early summer (I wrote a musing.io reply about that) most people consider Christmas to be the birthday of the "King of the Jews" (Mark 15:2). Was "Gus" mocking the Christmas holiday by calling himself "Polka King of the Midwest"? Maybe. It is something to think about.
Kate McCallister may have sold her soul to the Devil and not have known it. Just like most viewers of Home Alone may not realize that the Devil was one of the characters. But maybe you aren't as in favor of this fan theory as I am. It's like what a character in a movie released a few years later titled The Usual Suspects said, "...You never knew. That was his power. The greatest trick the Devil ever pulled was convincing the world he didn't exist."