There's no delicate way to say it: this guy is a slimeball.
Your first hint that Harry Lime's a scumbag? No, it's not the fact that he's played by Joe "Typecast As Lowlifes" Pesci. No, it's not the fact that he's casing a suburban house at Christmas. It's the fact that he has the same name as the villain of the uber-famous and uber-lauded 1949 film noir The Third Man.
Yeah. If you have the name "Harry Lime," you're either a low-rent burglar…or a black marketer selling watered-down penicillin to small children. No real gray area there.
Home Alone's Harry impersonates a police officer when we first meet him—we don't necessarily believe he's a burglar, but we sense there's something sinister going on. The way he smiles at Kevin oozes dishonesty…and it weirds Kevin out.
Harry stops by the McCallister house, acting like he wants to make sure that they're protecting their house while they take their trip to France. Kevin's dad tells him about their automatic light timers, immediately tipping Harry off to the fact that if their lights are on, it doesn't necessarily mean anybody's home.
When he smiles at Kevin so suspiciously, it reveals his gold tooth—which Kevin will recognize later on, and which Kevin's dad will eventually find lying on the floor of their house…
Harry covets the McCallister house, calling it the "silver tuna" and telling his partner, Marv:
HARRY: Look, that house is the only reason we started working this block in the first place. Ever since I laid eyes on that house, I wanted it.
Harry just wants to steal stuff—his motives aren't that complicated and he doesn't have another layer of depth to his personality. In a way, he's more humane, or more intelligent than Marv, since he finds Marv's taste for flooding the houses they rob to be "sick." But don't start thinking he's a saint or anything. He has one hobby, one passion: stealing.
Harry vs. Kevin
The first time they try to rob the McCallister house, Kevin turns on the lights at an unexpected moment, scaring them off. After Kevin recognizes Harry's tooth, Harry follows him down the street in his van. Next, Kevin frightens them away again by simulating a party at the house, using a cardboard cutout of Michael Jordan attached to a toy train and a mannequin rotating on a record player.
His third attempt to scare them away—using his Angels with Filthy Souls video—works on Marv, but Harry realizes something's up. He figures out Kevin's home alone…but Kevin overhears their plans to return.
So—especially compared to Marv—Harry's brains of the operation. He's the one who directs them, who picks the houses they're going to rob, and the one who orders Marv around. That being said, he's still not very bright.
Thinking kids are all idiots, he believes they can just walk up to Kevin's house and knock on his door, claiming to be Santa and an elf. Kevin's response? To immediately shoot Harry in the crotch with a BB gun. For Harry, things go downhill from there.
After getting burned in the head with a blowtorch, slipping down icy steps, burning his hand, and getting hit with a bunch of paint cans, Harry vents, yelling at Kevin,
HARRY: You bomb me with one more can, kid, and I'll snap off your cajones and boil them in motor oil!
It's not the nicest way to talk to a kid, but Harry's under some serious stress. His whole tough guy identity is falling apart.
Eventually, Harry & Co. manage to corner Kevin, and Harry prepares to start biting Kevin's fingers off. But Old Man Marley sneaks up behind him, and knocks Harry and Marv out with his snow-shovel. Poor Harry never gets to experiment with cannibalism.
Shortly thereafter, they're both getting stuffed into a police car. Kevin smiles from his window as Harry glares back at him. The message is simple and timeless: crime doesn't pay. Also, Harry's confidence in his own criminal genius is totally undermined, since he gets beat by a kid. It's a humbling fall.
Maybe Harry will re-evaluate his way of life, realize crime doesn't pay, turn to the good side? Nah. He's back in the sequel. You can't teach these comical burglar dudes anything.