“If you see something, say something” may be a good idea when it comes to national security, but it doesn’t work in some Philadelphia parks, where you may find angry men with hair-trigger tempers and poor impulse control.

That was the lesson Drew Justice learned. He wanted to be a good citizen. It cost him his life — and the cost to his accused assailant is minimal.

What is a life worth, anyway?

Justice and his fiancee, Kristi Buchholz, were walking their black shih tzu in South Philly’s Gold Star Park on Jan. 5 when they noticed a man allowing two dogs to run off-leash. Philadelphia law requires dogs to be leashed.

Justice, 38, called over for the man to leash his two dogs. Matthew Oropeza, 24, had driven the dogs to the park. Words were followed by a punch that killed Justice, according to a police affidavit obtained by my colleague Tommy Rowan.

Oropeza fled, but was arrested. He told police about the confrontation with Justice and also about an earlier verbal altercation in the park with another man who had complained about his dogs being unleashed.

Oropeza had two prior arrests for fighting, in 2013 and 2016. He pleaded guilty to the first and was put into a rehab program for the second.

Justice was acting as a good citizen and neighbor, but that can be dangerous.

Some years back, I was walking my dog in South Philly’s Marconi Park when I saw a guy whose dog was off leash and taking a dump on the grass.

“I hope you’re going to pick that up,” I called over in a neutral tone.

“What’s it to you?” he growled, taking a step toward me. We were about 20 feet apart.

“It makes all dog owners look bad,” I replied.

He shot me a dirty look, but that was the end of it.

It might not have been. It wasn’t for Drew Justice.

The police report showed that the medical examiner said the death was caused by “blunt impact injury to the head” and the manner was homicide.

But Oropeza was charged only with involuntary manslaughter, with a five-year maximum.

Five years for taking a life?

Pennsylvania law says that a death caused even by reckless or negligent behavior is involuntary manslaughter.

It falls into a crime category called “one-punch killings” that rarely carry a penalty as severe as they should.

The district attorney makes decisions about charges “based on the facts and the law as it applies to each case,” says DA spokesperson Ben Waxman. As I read the statute, murder in the third degree, which requires malice, also would fit.

Former Assistant DA George Parry tells me he can see “involuntary manslaughter” being a good call here, but “in the old days, we’d charge criminal homicide and let the jury hash it out, but we would not include first- or second-degree murder.” But the “old days” ended with Larry Krasner’s arrival in the DA’s Office.

What “one-punch killings” have in common is the absence of intent.

Is one life worth more than five years? Of course, but the law offers a loophole that shields the assailant from the consequences of even negligent and reckless actions.

It’s something to keep in mind if you get the urge to be a good citizen and get involved.

I hate to say it, but sometimes it’s smarter to zip it and look the other way.