Honestly? The guy had a knife, which might have injured one of the cops, maybe. US police are far too afraid for their own safety, and automatically reach for their gun when they think there might be the slightest minor chance that they could be in a little bit of danger.

If being in dangerous situations makes you open fire in a crowd of random innocent people, then you should not be a cop. Cops need to learn to accept the risks they signed up for, de-escalate, and protect the public before they obsess over protecting themselves. I know protecting the public is not technically their job, but opening fire in a crowded subway is laughably irresponsible, and should be an immediate fireable offense.