Sorry Kindler. I meant Harry's hatred of Marcone.
D'oh. That's on me. Pronouns, amirite?
Personally, I think his disdain and animosity for Marcone is for a few reasons. First: Harry works with the cops most of the time as of Storm Front (he'd have been bankrupt without them). Marcone has been making their lives hell. Harry's friendly with a lot of them—even vitriolic bros with Carmichael (whose Heroic Sacrifice in Fool Moon was, I think, the highlight of the book, especially when Carmichael tells everyone to do whatever Harry says, which is subtly excellent characterization for such a young writer like Jim was at the time).
Second, Marcone's business requires victims to be successful. Harry sees him as a predator, someone picking on the weak, kids notwithstanding.
Third, recall the total interaction. It starts with Harry being threatened and forced into the car. Harry responds to the threats with his normal flippancy. Marcone initially tries to offer him a wad of money, which Harry resents, because (let's face it) Young Harry regards himself the same way Don Quixote does (except they aren't windmills; they actually
are giants), which pisses him off a bit, even if he does think about it.
Fourth, Marcone threatens Murphy. She and Susan are pretty much Harry's only friends, and Susan is more of an acquaintance.
Then they soulgaze, and Marcone scares the crap out of him.
As a sidenote, I really, really want to know what Marcone saw, because he started treating Dresden with an awful lot of respect after that. One of my favorite Marcone scenes is when he chucks the knife into Will's bicep for sassing him, basically because Will isn't Harry. Their Enemy Mine relationship is a whole lot of fun.