A man lost his temper after a near-collision with a cab in Pacific Beach and punched two of its occupants -- one of them fatally, a prosecutor said Wednesday, but a defense attorney told jurors his client acted in self-defense.Sam Bonsu, 36, should be convicted of involuntary manslaughter and assault for punching Colin Costanzo in the mouth last May 24 and landing a blow to the temple of 25-year-old Nathaniel "Nat" Ward, who died five days later, said Deputy District Attorney Corine Miesfeld."This case is a senseless tragedy, not a fight at the beach," the prosecutor said in her opening statement.Miesfeld said Ward, Costanzo and two other high school friends were returning from a night of drinking and celebrating at a Pacific Beach bar when a gray Scion darted in front of the cab they were riding in.The defendant -- who was driving the Scion -- pulled up next to the cab as it dropped off the victims at Costanzo's apartment and confronted him, the prosecutor said.She said Bonsu yelled to Costanzo, "Hey dude, what was that all about? What the hell?"As Costanzo turned his back to pay the cab driver, Bonsu punched him in the mouth, splitting a tooth and putting a hole in his lip, Miesfeld said.Bonsu -- who at 6-foot-2 and 220 pounds was much larger than any of the victims -- punched the 5-foot-5 Ward so hard in the temple that it cracked his skull, the prosecutor said."That punch knocked him out cold," Miesfeld told the jury.After the scuffle, a female passenger in the Scion yelled to Bonsu, "Leave him alone. Let's go, let's go!" according to the prosecutor.Defense attorney Gary Gibson told the jury in his opening statement that the victims were all drunk and Bonsu sober at the time of the confrontation.None of Ward's three friends saw him get hit, Gibson said."They didn't see it, they didn't hear it," the attorney said.Gibson said Bonsu and his date were on their way to get some bottled water about 2 a.m. when he noticed the taxi cab speeding down the road.The people inside the taxi flipped Bonsu off and screamed obscenities at him, Gibson told the jury."That ticked him off a little bit because he lived there (in Pacific Beach)," Gibson said.Bonsu -- who had been out of the Navy for six years -- made a mistake by getting out of his car after his date said someone kicked the vehicle, Gibson said."He knew they were all hammered," the attorney said.Gibson said Bonsu punched Constanzo after the man walked toward his client and raised his hand. After that, someone hit Bonsu with a brown bag and he "hit whoever was closest," Gibson said.The attorney said Bonsu threw the blows because he felt surrounded and had to defend himself.On the witness stand, Costanzo testified that Bonsu approached and started yelling at him after the Scion came out from a side street and narrowly avoided hitting the taxicab."He said `Who flipped me off?"' Costanzo said."I told him, I don't know what you're talking about. You need to calm down," Costanzo testified.The witness said he turned his back on Bonsu and leaned into the cab to pay the driver."I started to stand up and I was hit from behind," Costanzo said.Costanzo said he was knocked face-down into the front seat and was "seeing stars" before getting up and realizing Ward had been punched to the ground.This is Bonsu's second trial. A jury last October acquitted him of second-degree murder and voluntary manslaughter.He faces up to nine years in prison if convicted.