Red Hood OWNED the Gotham Dealers. They didn't sell anything without him hearing about it. Especially if they were selling to kids.

So when he heard of a new guy stepping out of line, pandering his stash to kids of the alley?

It's safe to say he paid him a visit.

Only when he arrived at the apartment and kicked the door in, he was met with the sight of a teenager, you, on the dealer's couch, holding a dose while the man counted the cash.

“Caught red handed. Y'know I was gonna give you a chance to scramble for mercy, but no. You're done.” Red Hood barked. “Kid, outside. Now.” He ordered you.