HARRY: There's always a lot of burglaries around the holidays. So we're just checking the neighborhood to see if everyone's taking the proper precautions; that's all.
PETER: Oh, yeah. Well we have automatic timers for our lights, locks for our doors. That's about as well as anybody can do these days, right? Did you get some eggnog or something like that?
Ironically, Harry—who's impersonating a cop and pretending to be guarding the neighborhood—is actually a burglar himself! He's insinuating himself into their comfortable world, getting ready to strike…
HARRY: And that's the one, Marvin. And that's the silver tuna.
MARV: It's very G.
HARRY: Very G, huh? It's loaded. It's got lots of top-flight goods. Stereos, VCRs…
HARRY: Probably looking at some very fine jewelry, possible cash hoard, odd marketable securities… Who knows? It's a gem. Grab your crowbar.
Marv and Harry are covetous. They can't admire all this prosperity at a distance. It inspires them to go for it.
HARRY: What's so funny? What are you laughing at? You did it again, didn't you? You left the water running. What's wrong with you? Why do you do that? I told you not to do it.
MARV: Harry, it's our calling card!
HARRY: Calling card.
MARV: All the great ones leave their mark. We're the wet bandits!
Beneath Marv's stupid gimmick, there's a hard streak of destructive fury. It's not enough for him to plunder these houses—he needs to physically ruin them too, by flooding them.
HARRY: You're sick, you know? You're really sick.
MARV: I'm sick…?
HARRY: Yes, you are.
MARV: I'm not sick.
HARRY: It's a sick thing to do!
Harry seems to be a somewhat more rational person—he (evidently) doesn't want to inflict damage beyond theft, whereas Marv does. At the same time, Harry doesn't really care, and might just enjoy putting Marv down.
MARV: Maybe he went in the church?
HARRY: I'm not going in there.
MARV: Me neither.
Clearly, the burglars have a strong aversion to church. It's outside their comfort zone—a sanctuary where they can't follow Kevin (who's actually disguised himself in the Christmas manger).
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's particularly obsessed with the McCallister house. And why wouldn't he be? It's practically a mansion, and Kevin's an unusually wealthy hero.
OFFICER DEVEREAUX: Hey, you know we've been looking for you two guys for a long time. You guys are always leaving the water running whenever you break in, now we know each and every house you guys have hit.
MARV: Yeah. But remember, we're the wet bandits. The wet bandits. W-E-T.
HARRY: Shut up!
The extra dash of cruelty—flooding the houses—ends up being the very thing that totally incriminates Marv and Harry. They never managed to think logically.