Huge plot hole

To be honest George, you're about halfway between a troll and dumb.

If you're a troll, try harder. I actually enjoy a good troll post. Many trollers are in this for laughs, and many of those are indeed quite funny.

If you're dumb, the answer is that Buzz was being a tool and trying to scare Kevin so he made up the story about Marley.

If you're less than 10 years old, then you're forgiven.

Buzz is dumb enough to believe both that nothing bad could ever happen in their neighborhood, and that the old man next door is a serial killer. The only way Buzz could ever recognize any potential danger Kevin was in would be for him to actually be considerate of another person which doesn't not appear to be something he is capable of doing for anyone, let alone his youngest brother.

I think Buzz was just trying to scare his siblings/relatives, though when old man Marley looks up and they quickly hide maybe not.

You're right, I never thought about it.

There might be several areas of thought on this:

1.) It's mostly just an urban legend told around the neighborhood, and as we found out later, it was all just gossip. You can tell Buzz took pleasure in getting an audience, considering he enjoys being the center of attention. It's not enough that he's the favored son in the family, but he has to have all the attention too.

2.) Buzz is pretty dumb. Even by teenage standards, he's pretty stupid, especially if he believes in urban legends to be true. He's not creative when it comes to anything, even when bullying his little brother or spreading gossip about the neighbors.

3.) Buzz is a selfish ass. It's pretty obvious he only cares about himself and was very irritated in Paris when everyone was worrying about Kevin and not paying any attention to him. Plus, he could care less about about how "safe" Kevin was all alone back in Chicago. Heck, Kevin could have been killed by the Wet Bandits and Buzz would have just muttered "good riddance" without any remorse.

4.) Buzz's belief that their neighborhood was boring and safe is [sadly] a common subconscious belief among suburbanites who have never had any problems for a long period of time, and have never had any major disasters happen to them. They start believing that nothing bad ever will happen because it hasn't so far. All it takes is one disaster, be it natural, or man-made, and it totally changes your perspective on just how "safe" your neighborhood truly is.

Because Buzz was making up that stuff about old man Marley.

Buzz seems like the kind of kid who would believe the urban legends around his neighborhood but also believe it's safe. Growing up, those sort of urban legends floated around where I lived and yet, the neighborhood was considered safe. Kids don't usually consider the contradiction.