Of course he can start shooting. You are allowed to protect yourself and others with lethal force when your friends has just been possibly killed right next to you.
Is it reasonable for Nate to assume he was next? Yes. He was outnumbered, surroundedby dangerous thugs with weapons and he was on the floor. He should have wastedas many as possible, such a poorly written scene. Firing warning shots in the air? Only on a tv show. Those shots would come down a few minutes later and may kill someone unrelated and he'd be done for manslaughter. The academy would never in a million years tell anyone to do that and it seems the writers were pandering to the misguided public opinion.

(1) A cop doesn't shoot to wound. Cops are trained to shoot center mass and be lethal.
(2) Sammy was outnumbered. If he starts wildly shooting, he will get killed too.
(3) Yes, Nate was attacked but Sammy can't just start shooting people because of that. If he did, and he survived, he'd be charged because the gang members at that point were not being a threat to his life.
(4) He doesn't have enough bullets for all of them.
__
Welcome to the O.C., bitch. -
Luke Ward
,
The O.C.

Because there were more people than bullets in his clip?
What an odd question.
Due to the lack of moderators, trolls can ruin the IMDB message boards. Don't feed them.

Ummmm if you watch the episode again he does actually shoot a gangbanger getting too close to Nate's body.
If you are not willing to give up everything, you have already lost

Yeah, but they were only a few feet from him. He can only shoot so many before the rest overpower him. Bare hands can be just as lethal as guns. Especially when there's around a dosen pairs of hands.

They didn't seem to have been armed (which maybe isn't very plausible), otherwise they surely had drawn their guns, instead of trying to just grab Sammy. So if you shoot one or two (preferably the ones with baseball bats or rods) you'd already be a bit "safer", no?

there's plenty of kooks out there. data travelers, electro wizards, techno anarchists!

Because before the dead gang member had hit the ground, every single one of the orhers would have been on them and they'd both be killed.