I read the book! (Sort of)

The murder of Adrian Mellon marked the return of IT, after its 27 year hibernation. It also marked the beginning of the killings that would lead Mike to call the rest of the Losers Club back to Derry.

I intend to live forever.
So far, so good.

The book was written in the 80s. The murder is based on a real murder of a gay man that happened in Bangor a few years before, not far from King's house. It was a huge topic of the time, and King thought it fit with the violent themes of the book. He needed to write about it.

In the book it's one of a few chapters that is a bit redundant, but it does introduce a few points that will be elaborated later on. It gives an interesting perspective on the grand picture. The spoilers here are fairly minor, but they answer your questions. The book goes into the nature of It much deeper than the tv series, so just in case you want to be totally surprised I'll hide them.

* It serves as the prologue to the adult section of the book, hence an adult murder
* The hatecrime is the big event that woke It and began the new cycle, just like the storm of '57.
* The relationship of the town of Derry with It and how they ignore It, and the violence It inspires in the town. King subtly establishes that the town of Derry and It are linked to each other. They're pretty much one and the same

It doesn't feed on just kids. It just prefers to feed on kids, because the fear of a child is so much more basic and thus easier to evoke.