Cracked had a good rundown of the messy details.

In the early '90s, Heemeyer got involved in a campaign to bring legalized gambling to the city, even launching his own newspaper to promote it. Patrick Brower's paper had come out against the measure, which was eventually voted down. That's the first time the local press and government wound up on Heemeyer's bad side.

Around that time, Heemeyer bought a patch of land for $42,000 and opened another muffler shop there. Several years later, a group looking to open a concrete batch plant offered to buy the land for $250,000. Heemeyer reportedly agreed, then later bumped the price up to $375,000, and maybe went as high as $1 million. Instead, the buyers went to the city to rezone the land around the shop and build there instead.

Heemeyer attempted to start a grassroots campaign to fight the plant. The problem was that the new plant would cut off customer access to his shop completely -- he apparently bought the bulldozer to create a new road, but permission for that was denied too. He lost his battle and was forced to sell the shop, at which point he immediately started building a Killdozer. The rampage would come a year and a half later; it was hardly done on impulse.

Did the zoning dispute go badly for him? No doubt. Did he have multiple chances to avoid that outcome? It seems like it. Did the city leave him no choice but to do what he did? Well, most schools of morality posit that even in the most bitter business disputes, there are always non-Killdozer options.

...

In addition to the newspaper, Heemeyer tore through the concrete plant, the town hall, a hardware store owned by another man he had a dispute with -- he'd made a list. That's where the noble vigilante story comes into play. He wasn't out to hurt anybody, they say, just to do to others what they'd done to him.

But as Piechocki points out, the lack of a body count was down to pure luck. "When he attacked the town hall, the library is in there, and my son was in that library at that time. Gambles, the store he got stuck in, what happened if somebody was stuck in there?" In fact, most of the buildings were occupied right up until the inhabitants ran away screaming that a Killdozer was coming their way.