Apotheosis/Atlas/Backstory/Watch her get raped
You decide not to intervene in any way whatsoever. Maybe you're scared of confrontation; maybe you happen to be into watching other men fuck girls while you stand back; who knows?
Either way, since time has passed for them to proceed forward, they waste no more time. They lead her a little further down the alley, finding a little garbage container that is just about perfect height for any adult to easily sit on without needing to jump or climb on top of it. You can see that the men have stripped her pants, and that one of them is sitting her on the container. You can also clearly see the big burly man drop his pants and begin fucking the girl. However, there is one huge problem here; you can't see a damn thing. All of the action is hidden from with, with only her legs and the big man's bare bottom visible from this angle. You still can't hear what they're talking and laughing about, but you can hear occasional soft moans, suggesting that she's enjoying getting fucked.
You take an instinctive step closer, but your timing is horrible. One of the men turns towards you, probably to make sure nobody's checking on what's happening, and he sees you.
"Get out", he says. "There's nothing to see here."
"Wait, what?" says the other one who's not getting any action, yet. "We've got company?"
"Yeah, this kid's been eavesdropping on us", says the thug, unsheathing his knife.
"Who cares? He's just a kid", the other one says, letting out a laugh. "He's probably horny because he's never seen a girl up close."
"Hehe, I'm fine with letting some kid watch me bone a whore", says the thug fucking the girl. "It's probably better than letting him cause a scene."
"Alright, you heard the man", says the armed thug, walking up to you. Seeing him approach you while armed causes you instinctively to lay your hand on your weapon. He does realize this, and it causes him to approach more cautiously, but also with an angrier look on his face. "Don't resist, kid", he says, extending his hand to you. "Give me your weapon."
"N-no." Your immediate reaction is to resist without thinking. A true soldier surrenders his weapon to anyone.
"Don't worry", he says. "I'll give your toy back to you after we'll done. Do what I say, and nobody gets hurt."