Cristobal Palacio, on trial for killing his ex-wife’s new husband, couldn’t hold back the tears as his 10-year-old daughter testified about what she saw four years ago.

From inside her mother’s van, parked outside Palacio’s Kendall home, she and her twin brother saw their stepfather shot dead.

“He was smiling,” the girl said of her father that day.

The girl, identified only as E.P., testified Tuesday on the first day of Palacio’s first-degree murder trial in Miami-Dade County Circuit Court, where he also is facing charges of child abuse.

The facts of the case are clear: Palacio, 46, called police after emptying his clip. He fired six shots, two of them into the victim’s back.

He confessed to gunning down his ex-wife’s new spouse in self-defense. Court Judge Leon M. Firtel warned the jury before witnesses took the stand that the case was not about Florida’s stand your ground law, but the claim will be left to the jury.

Defense attorney Michael Walsh said Palacio was “exercising the most basic human right” after Paul Winter threatened his client several times, even putting a gun to his head.

Prosecutor Christine Hernandez painted Palacio as a revenge-seeking ex-husband who calculated the murder of Winter, then 42, out of spite.

“He began to torment” his former wife, Hernandez told the jurors Tuesday. “But she did move on.”

Jennifer Winter married her new husband three days after divorcing Palacio.

After the previous marriage fell apart, the bitterness remained. Most of the disputes involved custody of the two children.

On her 37th birthday, Jennifer Winter went to pick up the twins from school. Palacio was already there.

After a brief exchange, Winter took the kids home.

To avoid another legal quarrel, she called Palacio to make sure she wasn’t taking the kids out of turn.

“Bring them back tonight,” he said, according to the state prosecutor. She did.

That tension, prosecutors said, led up to the deadly events of Oct. 17, 2008.

Paul was driving with Jennifer in a minivan to drop off the two 6-year-olds, and they parked in the street outside Palacio’s home.

The girl, now 10, told jurors Tuesday that she and her brother got out of the van to knock on Palacio’s front door. When he didn’t answer, their mother knocked, with no response. The three went back to the van.

When Palacio finally came out of the house, she said her stepfather got out of the van and walked around the front. That’s when the girl saw Palacio shoot Winter.

Jennifer Winter will testify when the trial continues Wednesday.

Palacio’s defense attorney said Paul Winter charged at him in the driveway.

“Paul Winter was the architect of his own death,” defense attorney Walsh said.

As for the smile, Walsh said Palacio has a nervous tick that sometimes makes him look happy.