Which makes me wonder why he was only charged with the latter; it's clear he committed the former.

The story here is not as simple as the article sounds.

The short version is Timothy Newman, the man who ended up dead, had armed himself with a gun, gone to Campbell's property with the intent of "adversely possessing" it, and draw his own gun on Campbell before Campbell fired at him.

Now this probably would not have made it to a trial, had not Campbell said to multiple people in the weeks before that he was going to “put Newman down” the next time they met.

There was a murder trial. The jury at this trial were instructed to decided between preplanned murder or innocent, did not believe either was correct, and returned no decision, resulting in a mistrial.

In lieu of a second trial, the 20-year suspended sentence was worked out between the prosecutors and the defendant's lawyers. As part of that Campbell paid Newman's wife one million dollars, and Campbell was never to go on the disputed property again (effectively giving up control of it).

The wild west is a crazy place, and justice is not always obvious.

"last year he finally pleaded guilty to negligent homicide."

It sounds like they had enough to charge him with more (how much more - who knows?).

He reached a pleasant deal with the prosecutor, he wasn't convicted by a jury.