Special Agent Christopher Deedy said his training taught him to drink responsibly: no more than one drink per hour he told jurors on Tuesday.
He said by 2 a.m. Nov. 5, 2011, he'd had about four beers in a five hour period.
He said he immediately noticed when a loud, boisterous Kollin Elderts walked in to a Waikiki McDonald's.
Deedy said Elderts started harassing and swearing at a customer, before Deedy decided to step in.
Deedy said the moment Kollin Elderts walked into the Waikiki McDonalds, Elderts was loud and boisterous, before he said Elderts and his friend Shane Medeiros began bulling customer Michel Perrine.
Then Deedy said, the two started accusing Perrine of "staring them down."
"When I addressed Mr. Elderts and then heard more threatening language addressing Mr. Perrine, it was now coming from two directions. So, I turned to Mr. Perrine and asked him if he was OK, but he was just staring off into space," said Deedy.
"As Mr. Perrine sat down, Elderts said something to Perrine like what the F is your problem? You wanna beef?" said Deedy.
That's when Deedy said he walked over to Elderts seated at a table and injected himself into the situation.
"I said to him something like nobody wants to fight here. Prior to reaching for my credits, Elderts said 'I'm gonna F you up.' At that point my mind was that it (things) were already in motion and it may escalate to a level of force here," said Deedy.
"So, I identified myself verbally. I told him I was a cop and that he could be arrested and I reached back and opened up my (law enforcement credentials) and displayed them to him," he said.
But it was move Deedy says angered Elderts.
"When opened my credentials I looked up Medeiros who was staring right down at them, and Elderts responded with you won't arrest me. You gotta gun? What you gonna shoot me? So F-ing shoot me," he said.
It's a threat he said his training taught him to respond to and a situation he was now committed to.
"Why not run out the door to avoid any further contact with Mr. Elderts?" said attorney Brook Hart.
"For me, seeing a volatile individual, as a law enforcement officer, to simply walk away, would be totally irresponsible for me," said Deedy.
But it would be a situation that would continue to escalate.
Deedy said it was Elderts who then moved forward, with one customer stepping between them, before Deedy said he deployed a thrust kick, a common move taught in training, as a way to avoid a verbal fight from turning into a physical one.
"The way he was telling me I was going to need my gun, gonna get me first, go ahead and shoot him, touching his waist band, a reasonable officer would believe this person had the intent, the ability, and the means to do serious injury," he said.
Earlier in the trial, another special agent testified that he warned Deedy to watch out for the term 'haole' as a negative or racial term.
They were both in Hawaii to work the Asia-Pacific Economic Cooperation or APEC.
Deedy said that conversation did not inspire him to carry his gun.
He said he would have carried it anyway, since he considered himself "on call" that night.