Release it anonymously!
I don't think it's unambiguously wrong from a moral standpoint.
I think it is. I have 5 million passwords belonging to ~15 million users. I'd be directly harming some proportion of those users. That makes it wrong in my mind.
Additionally, there is some malice at play - somebody stole these, and right now investigators are trying to find that person/people. If I release it with the name of the company in question, those people are going to be tipped off that others know what's going on and will likely be more difficult to find.