One reason that there is so much vitriol being directed toward him right now is that many of the things he and his brother have been doing for decades has recently been coming to light. In large part, people are reacting strongly to his death because the harm he caused has directly affected many of the people lashing out. Here's a short list of some of what he's done, but the core cause of all this is that the brothers seem to have held Greed to be their sole focus.
Koch Industries, the company he and his brother ran, is a major source of pollution in the US. It is a multinational corporation that deals with everything related to oil including pipelines, fertilizer, mining, and other related activities.
Both brothers actively spent millions of dollars to fight against laws that would limit their pollution.
Spent money to actively lobby against public transit in a number of cities, because public transit would mean fewer people would need to buy their gas.
In fitting with the above, they put tens of millions of dollars into lobby groups that spread misinformation claiming that climate change is not real. This directly affects our ability to try to mitigate the effects of climate change.
The people celebrating are politically left of center, and the Koch brothers were quite directly responsible for funding the campaigns of some of the furthest right of center politicians our country has ever seen. Including the current president. They created political networks of other rich people and spent hundreds of millions of dollars working to move the political conversation further to the right, in support of policies that have significantly widened the wealth gap in America, weakened our social programs, weakened our environmental protections, and generally caused a great deal of harm. For example, during the 2016 election, they spent $300 million of their own money and another $600 million from their friends on such causes.
In large part because of their actions and the actions of those like them, the American political landscape has become extremely partisan. It has become more and more difficult to feel anything but anger toward "the other side", for both sides. So, the passing of one of the two brothers means "their side" loses a major source of economic and policial influence that the political left sees as directly harmful. You said that he "seemed like an average republican billionaire", and in a way you're correct. He and his brother are just among the most extreme and politically active of that kind. They're often spoken of as being examples of the kind of corruption that can be caused by excessive wealth. So much so that there is a subreddit dedicated specifically to tracking the harm of those two brothers. /r/KochWatch
In that kind of environment, and with having spent decades seeing the harm he caused, it becomes difficult to feel anything but anger toward David Koch. Even in his death. Intellectually, I know that feeling anger at a dead man does little more than harm my own equanimity. But it's still hard to feel another way. I'm sorry if this got rambly. I am one of those with strong feelings about the Kochs. It is something I am trying to fix in myself.