What is the Laura flaw?

One afternoon I put on the movie Laura and as soon as Clifton Webb appeared, I said to myself, "he did it," and promptly fell asleep.

The movie starts with a detective out to find what happened to Laura, who is already dead or soon to be dead (can't remember right now). Clifton Webb is the resident fruit-about-town who offers to show the detective around. Webb knows everyone and is eager to help.

The Laura flaw means that the person who offers to give the protagonist help is the bad guy. This plot device pops up in many movies: the cop who's asked by his boss not to tell anyone about the evidence and to meet him later in some secluded place then finds out his boss it crooked; the odd character who hangs around too much; the best friend who is always there to help...

Whenever I see this happen in a movie, I automatically think how this "helper" could be the bad guy. Who is Keyser Soze? I know who.
Is Harrison Ford guilty in Presumed Innocent? I know.
Did Clifton Webb kill Laura? You tell me.