Read the rest of the article. He tried to deescalate the situation before firing the fatal rounds. He didn't just "execute" him outright. His prior comment during the high speed chase could easily be argued as a heat of the moment type a thing.
Facts are, the prosecution failed to present any tangible evidence that the gun was planted. Their whole case hinged on this gun and the vitcim's lack of DNA being found on it. I was able to dig up a few true crime cases where a person unquestionably handled a deadly weapon (based on other evidence), but did not leave any identifiable DNA on said weapon (one case had a mixture of DNA from two unidentified males that excluded the suspect). Thus, the prosecution's contention that this lack of DNA proved the gun was planted is very weak circumstantial evidence.
What both sides agree on is that the victim was engaged in a heroin deal just before the high speed chase, and we all can admit that it would be highly unlikely that such a man would be unarmed during a transaction involving drugs like this.
The cop broke his own department rules, he disobeyed direct order he lied about the events of that night. But yeah lets give him the benefit of the doubt.
Criminal defendants are entitled to the benefit of the doubt. It's the prosecutors job to remove that doubt. ---