Aren't we all really the same underneath it all? Maybe—but Harry's a jerk, and Marley's a nice guy. That's Home Alone's way of seeing things.
At first, we see Harry's identity in reverse: in reality, he's a robber, but he masquerades as a cop to learn about the kind of security precautions the McCallisters are taking during the holidays. It's a ruse to get behind their defenses.
At the same time, early in the movie, we hear from Buzz (an obviously unreliable source) that Old Man Marley is a serial killer who murders people with his snow shovel an turns them into mummies by putting them into the salt he uses on the sidewalk. Now, if we step back for a second, this is obviously false—and Harry seems a little sinister as the cop.
But, in Kevin's mind, he sees Marley as a serial killer, until he learns better.
The classic take-away lesson is, "Appearances can be deceiving." The initial impression is the opposite of the truth: Harry isn't a cop, he's a robber; and Marley is actually a kindly old man who really just likes helping people get their sidewalks cleared.
Part of maturity for Kevin—even though he's only eight years old—involves learning to see the difference between reality and appearance. He's able to pierce behind this veil of illusion to decipher what's really going on, and it benefits him. By realizing that Marley's one of the good guys, Kevin gains an ally—an ally who saves him from Marv and Harry when they finally catch him. They're brothers in arms.