By all accounts, this guy gets a raw deal. (Nothing shouts "raw" like "your lungs full of saltwater and left to rot in a dry river bed.")
Hollis has been a good guy all along, even though the local farmers think that he's trying to steal their water. In reality, he's trying to protect them: they want a new dam which they think will re-route water to their fields, but, as chief engineer at the water department, Hollis won't approve it.
This is because he knows that the dam is actually a scam (we're poets). It won't actually divert water to the farmers—it's a scheme for Noah Cross to divert the water to his own lands, steal it, and then sell it to them for a price. Hollis, who co-founded the water company with Noah and is married to his daughter, is standing in the way—and Noah has him murdered for it.
Clearly, Hollis is a man of principle. He thinks the water department should be the public utility it's meant to be…and not the private property of a guy like Noah. But, aside from a speech in which he says he won't approve the dam, we don't hear much from him, even though his murder is the incident that sparks to Jake Gittes' journey of discovery.
Unwittingly, Jake helps Noah's designs by framing Hollis for adultery—which sets the stage for Hollis's murder (which looks like a suicide because of the adultery photos). In reality, Hollis was just visiting Evelyn's daughter, Katherine. As Jake discovers at the end, Noah personally drowned Hollis in the saltwater pool in back of Evelyn's house, eliminating his rival for good.
What's that saying again—nice guys finish last? Yeah. Poor Hollis.