Smith, a defensive end who played for the Saints from 2004 to 2012 and helped the team win its only Super Bowl, was fatally shot in a road-rage incident.
"There's no winners", Payton declared, recalling how he watched Hayes being found guilty.
Buras said the sentences will be served concurrently, and Hayes will receive credit for time served.
A prosecutor is calling for maximum punishment for the killer of New Orleans Saints star Will Smith.
Following trial in December, a Jury declined to slap Hayes with a verdict of second-degree murder, which would have meant a mandatory life in prison.
Formal sentencing is expected later this week.
Hayes and his attorneys had argued in the Orleans Parish Criminal District Court trial that he fired his gun in self-defense, claiming that Smith, 34, was reaching for a gun he kept in his auto.
But New Orleans District Attorney Leon Cannizzaro soon made it clear he believes Hayes deserves a 60-year sentence.
Wearing a wild mane of hair and a bushy beard, Burnside spoke clearly and confidently but appeared near tears at times and twice let slip a profanity.
Cardell Hayes, the man convicted of shooting and killing former Saints player Will Smith has been sentenced to 25 years in prison, Judge Camille Buras ruled Thursday.More news: Mourinho relying on Rooney amid United's injury crisis
"I think that Judge Buras took everything into account", an attorney for Hayes, John Fuller, said. "Just because people are different from you, doesn't make them liars", he protested. He also acknowledged that he didn't witness the killing, and the prosecutor poked other holes in his testimony.
Burnside, who has owned guns and served in the military, said he heard "baps" from a smaller weapon, then "booms" from a larger one.
But Assistant District Attorney Jason Napoli noted that Michael Burnside said he doesn't own a clock or a calendar, and couldn't be certain of the time and date of the shooting he heard. In fact, there was another shooting in the same area about 45 minutes before Smith was killed, Napoli said. This would support the defense argument that Smith also fired a gun at the scene and that Hayes fired in self-defense. The defense has noted the absence of any serious criminal record and has said Hayes feared for his life when he encountered a drunken, belligerent Smith in a traffic dispute past year.
Hayes, who was convicted of manslaughter and attempted manslaughter, insisted at trial that he shot in self-defense. And later, after again insisting that he acted in self-defense as Smith fired a gun at him, despite trial evidence to the contrary, he referred to the Smith family, saying, "I apologize for their loss".
The judge said Hayes' formal sentencing would be delayed to at least Friday, but in preparation, she began taking testimony, hearing first from Smith's aunt, who helped raise the Ohio State and NFL football star.
Dawn Mumphrey also begged for the Smiths to forgive her son, 29-year-old Cardell Hayes, as she testified at a pre-sentence hearing Thursday.
Saints coach Sean Payton and Racquel Smith testified during the sentencing Wednesday, remembering the late Smith for his contributions on and off the field and devotion to his family. The records don't spell out what the evidence is. But Hayes may still be locked away for a very long time if prosecutors get their way at a sentencing hearing Wednesday, April 19, 2017.
That might push a final decision on sentencing to later in the week even if Buras rejects the motion.
Burnside sand the Orleans Parish District Attorney's office "thinks I'm insane and I don't care". That would include the maximum 40 years for Hayes' manslaughter conviction, followed by another 20 for attempted manslaughter in the wounding of Smith's wife.
Hayes was convicted of the crimes in December. Prosecutors said Smith went to his auto to retrieve his weapon only after getting shot and died in his vehicle before he could get his hands on it.