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He witnessed George Floyd's murder. Here's what he hopes people understand.

"I don't think most of America understands," Donald Williams II said of the trauma he still experiences. "They kind of see it and hear it, but I don't think they fully understand."

When Donald Williams II, a professional mixed martial arts fighter, took the witness stand in Derek Chauvin's murder trial in Minneapolis in March, it had been at least two years since his last fight.

But Williams, a prosecution witness who had watched helplessly as George Floyd died under Chauvin's knee last May, relied on his fighter instincts.

Williams had mentally prepared himself to enter a different kind of ring — a courtroom. That was evident in his calm responses to contentious questions from Chauvin's attorney, Eric Nelson, who tried to portray Williams and other onlookers who had shouted at Chauvin to get his knee off of Floyd's neck as an angry mob.

"You can't paint me out to be angry," Williams confidently told Nelson in a notable exchange from the early days of the trial.

Donald Williams answers questions during Derek Chauvin's trial in Minneapolis on March 29, 2021.Court TV via AP Pool

But that self-assured response was preceded by months of anguish — a pain that lingers even now, a year after Floyd's death, said Williams, a lifelong Minneapolis resident.

"I don't think most of America understands," he said of the trauma he still experiences. "They kind of see it and hear it, but I don't think they fully understand that the people that were actually involved, the things that they have to deal with for the rest of their lives or be recognized for for the rest of their lives — you know, someone actually losing their life — that is tough for everybody in the circle."

Williams, 33, said he has endured many sleepless nights consumed with thoughts of Floyd, as have other bystanders.

A police body camera shows bystanders including Donald Williams, center in black, as Derek Chauvin pressed his knee on George Floyd's neck in Minneapolis on May 25, 2020.Minneapolis Police Department / via AP file

Witnessing Floyd's murder prompted him for the first time in his life to see a therapist, whom he still speaks with regularly. For several months, his therapist was the only person he talked to about the pain he felt from that day, he said.

He also avoided the widely viewed bystander and police body camera videos of Floyd's detainment that were repeatedly played in court during the trial.

It was only during the trial that Williams heard for the first time audio of his desperate plea to 911 the day of Floyd's death.

In a recording of the emergency call, Williams could be heard yelling at the officers, "Y'all is murderers, bro!"

He testified that he had "called the police on the police" after Floyd was taken away in an ambulance and told the 911 dispatcher that he believed he'd witnessed a murder.

Despite the passage of time, Floyd's death continues to ripple outward, touching different facets of Williams' life.

Donald Williams in North Minneapolis on May 11, 2021.Drew Arrieta / for NBC News

"It affects my financial situation. It affects my kids. It affects everything that I have going on," said Williams, also an entrepreneur, whose businesses offer landscaping and snow removal services.

The trial itself was in some ways a testament to his resilience. Nelson, the defense attorney, made repeated attempts to rile and discredit him.

"That tactic, it was appalling to me because for him to try to paint me as an angry Black person and to try to get a rise out of me was pretty disappointing," Williams said. "But it just really made me understand that it was a championship fight and that it was an opponent in front of me trying to break me, and I just kept my composure because I've been through this before as a Black man."

Williams was at home with his son and daughter when he learned Chauvin, a former Minneapolis police officer, was convicted of second- and third-degree murder and second-degree manslaughter. Well before he testified, he said, he had had conversations with his children about the disparate treatment Black people receive from police, which he said he has experienced and witnessed growing up in Minneapolis.

They had prayed together before the verdict was announced.

Donald Williams smiles during a news conference after Chauvin's conviction, on April 20, 2021, in Minneapolis.John Minchillo / AP

"It was joyful and just, I guess, sad at the same time," Williams said. "We were just thankful that the verdict was what it is."

"I wasn't necessarily surprised," he added. "But I wasn't that severely trusting in the system."

Though he has not spoken to any of the other bystanders since last May, Williams said he knows they share a collective pain.

"That's why I just hope that everybody is doing well and is able to move forward," he said.

Williams, too, is looking to what's ahead.

He had been fishing with his son at Valentine Lake earlier in the day last May 25 before he went to Cup Foods to get something to drink and happened upon a life-derailing moment.

After spending a year under the weight of trauma, Williams said that on the anniversary of Floyd's death, he may take another step toward returning to the life he once enjoyed.

"Maybe just be with family again," he said of his plans for that day. "And try to get some fishing in since I only went once last year."

After spending a year under the weight of trauma, Williams said he may take another step toward returning to the life he once enjoyed on the anniversary of Floyd's death.Drew Arrieta / for NBC News

One place Williams' presence has been missed is Northside Boxing Club, a nonprofit in Minneapolis where Williams once trained and mentored children and young adults.

"He trained hard, had a disciplined life, had the right discipline that we're trying to bestow on the youth there," said Ryan Burnet, the founder of Northside Boxing Club.

Williams has fought professionally for a decade. He tore his ACL during his last fight in 2019 and is not actively training. He hasn't been to Northside Boxing Club in a year or so, but Burnet said he is always welcome.

"He knows that whenever he wants to come through there he has a home, and we would love to show him the love he needs and deserves," Burnet said. "As much as he would like to be there, he would be welcome. I think he'd be a continued asset to the gym and to the youth of Minneapolis."