In the aftermath of the May 2016 police shooting that left 23-year-old RJ Williams dead in Weirton, West Virginia, police officer Stephen Mader was shocked, nervous and scared.

“I was at a loss for what to do,” he told the Guardian.

Mader’s reaction was not to the shooting itself, which he feels he handled with poise and calm. In fact, he did not fire his weapon. It was a fellow officer’s gun that ended Williams’ life.

Mader was dumbstruck to have been handed a notice of termination, which indicated that his failure to shoot Williams, a young black father who was mentally ill, represented grounds for dismissal.

“It was just a right hook out of nowhere, right to the face,” said Mader, who grew up in the small town of Weirton before becoming an officer there.

“Essentially they fired [Mader] for respecting RJ Williams’ constitutional rights,” said Joseph Cohen of the American Civil Liberties Union of West Virginia.

The notice of termination accused Mader of “negligence”. “[Patrolman] Mader’s failure to react left himself and those around him in grave danger,” it read.

On Wednesday, Mader filed a lawsuit against the city for wrongful termination. “When a police [officer] exercises restraint – and sometimes we don’t see that as much as we like to – that’s something that should be praised rather than punished,” said Mader’s attorney, Tim O’Brien.

When a police [officer] exercises restraint, that’s something that should be praised rather than punished Tim O'Brien, Mader's attorney

The suit alleges that the city, “in a flawed effort to buttress the other officer’s use of deadly force, wrongfully terminated Mr Mader’s employment. When that termination came to light in the local press, the city then engaged in a pattern of retaliation designed to destroy Mr Mader’s reputation.”

As police responded to a disturbance at Williams’ home early on the morning of 6 May 2016, Mader was the first officer on the scene. When he arrived and confronted Williams, he said, the man was keeping his hands behind his back. Mader commanded Williams to show his hands and when they came down to his sides Mader saw he was holding a pistol.

Mader drew his weapon. Williams allegedly screamed: “Just shoot me!”

“I said, ‘I don’t wanna shoot you, brother, just put down the gun,’” Mader recalled. “About that time two more cruisers arrived. At this point he starts to wave his gun at me and the other officers, and within seconds of the other officers getting out of their cruisers there were four shots fired.”

Williams’ gun, investigators would later learn, was not loaded.

In the termination letter, Mader’s superiors argued that he failed to respond to the threat. “The unfortunate reality of police work is that making any decision is better than making no decision at all,” it says.

Mader counters that he did make a decision – that he decided, based on Williams’ body language and apparent mental state, that he did not present a threat and that de-escalation was the best way to proceed.

The unfortunate reality of police work is that making any decision is better than making no decision at all Mader's termination letter

Mader had a degree of familiarity with life-and-death situations. A former US marine who saw service in Afghanistan, he said he brought some of that mettle to his job as an officer.

“He wasn’t angry,” he said of Williams, “he wasn’t aggressive, he didn’t seem in position to want to use a gun against anybody. He never pointed it at me. I didn’t perceive him as an imminent threat.”

Mader’s commanding officers essentially argued that such an evaluation was not his to make. “No officer was ever trained to deduce the intention of a suspect,” the termination notice reads.

O’Brien disagrees. “If the emphasis is, ‘You must use deadly force if you can use deadly force’,” he said, “then you are taking away that critical discretion that every police officer must have.”

Despite the fact that the termination letter focuses most of its attention on the decision not to shoot Williams, department officials subsequently told the Pittsburgh Post-Gazette the incident was “not a primary factor in his termination”.

Officials cited two other incidents involving Mader. Only one is mentioned in the letter, in just two sentences.

By contrast, on the Williams shooting the letter reads that Mader “should be dismissed from employment … due to to negligence on his part during the incident that occurred on 6 May 2016, in which a fellow officer had to react and unfortunately take the life of the suspect”.

After Mader’s termination, officials said Mader “froze” during the incident. They also called him a “disgruntled employee” and a “bad cop”.

According to the suit, Mader is seeking damages in excess of $75,000. The city of Weirton and the Weirton police department did not immediately respond to requests for comment.