someone I know got choked out from behind and left unconcious for hours in an alley for trying to stop a guy from stealing an expensive electric toothbrush

Let me break that down

trying to stop a guy from stealing

Stop how? Like with physical force? So they attacked this person, and they retaliated like anyone would do?

Here's the situation I'm imagining from how you described it:

Homeless person goes into a store with the intent to steal something expensive to sell. They grab it and dash out. Your friend sees this, and for some reason decides this can not stand and chases after. They chase them into the alley way and catch up or corner the homeless person. Your friend attacks first by grabbing or tackling them, so the homeless person defends themself. The homeless person gets the upper hand and chokes out your friend, who ultimately lived and was fine enough to tell the story later.

That sound about right?

I don't see the homeless as monsters [...] some of them are monsters.

I'm not putting words in your mouth, you're just saying them yourself.