Moreover, he was given a lawful government order NOT TO BE DEPORTED to El Salvador.
This is one of the key issues that is unresolved. It matters not a whit if he killed 15 people (which he didn't). It matters not at all if he's in a gang (he's not). None of that addresses the lawful government order to not be deported to El Salvador - which the government violated.
The government admits it made a mistake.
These are the only two points to discuss right now. His prior history, beyond receiving an order from the US government saying he is not to be deported to El Salvador, has no bearing at this time.
If we clear up those two issues, we can discuss if we as Americans want someone like him in our society. But we ain't there yet.