If the weekend slaughter in Chinatown, where one homeless vagrant allegedly beat four others to death, shocks the conscience of decent New Yorkers, that’s a good sign. Now the goal must be to instill sanity — and common sense — in public health and criminal justice policies.

Because instead of making the streets better, the city is on a course to make them worse. And more dangerous.

The weekend horror proves again that the decision to empty the state’s mental hospitals was one of the great social mistakes of the last 50 years.

But as Stephen Eide points out in Tuesday’s Post, deinstitutionalization wasn’t a one-off. It continues to happen today, despite being an obvious disaster.

Randy Santos, charged with four murders, is the latest in a long line of those who qualify as Exhibit A. Santos, 24, has been arrested repeatedly for unprovoked violence and was evicted from a homeless shelter for attacking a man there.

His family kicked him out of their Bronx house and neighbors said his mental illness grew worse after he started smoking crack cocaine.

It was no secret he needed serious care, most likely in an institution. Instead, he got revolving-door treatment in police precincts, courts and hospitals, out of sight, out of mind, until he allegedly committed a heinous act that cannot be ignored.

Yet while Santos represents the dangers of untreated mental illness, City Hall accepts no responsibility. Mayor Bill de Blasio, who funded his wife’s pet project of combating ordinary depression with $1 billion, most of it wasted, argues that reforming the mental health landscape means starting from scratch.

This is pure bunk, typical of his reign of error. He acts as if New York was a hellhole when he became mayor.

In fact, the opposite is true. It’s becoming more of a hellhole because of him.

And now de Blasio wants to repeat the failed deinstitutionalization movement. His bid to close the Rikers Island jail and prison complex is little more than a scary replay of mental hospital closures.

The plan to replace Rikers with four smaller jails, one in each borough except Staten Island, smacks of the same snake-oil pitch used to close hospitals.

Then, too, the civil rights of the confined were part of the argument for dramatic change. Both times, the public was promised more community-based services that would offer better, more humane treatment than larger institutions.

The history of the hospital debacle, driven in part by court decisions, was neatly summed up in a 2013 article in the AMA Journal of Ethics, which said, “Three forces drove the movement of people with severe mental illness from hospitals into the community: the belief that mental hospitals were cruel and inhumane; the hope that new antipsychotic medications offered a cure; and the desire to save money.”

The author, Dr. Daniel Yohanna, added: “It has not worked out as well as expected on any of the three fronts.”

Talk about an understatement.

There is no compelling logic behind the move to close Rikers, only slogans and an unwarranted faith in the universal goodness of human nature. So-called reformers see inmate violence there not as proof that the inmates are dangerous, only as proof that Rikers itself is the problem.

Having failed to dramatically improve Rikers, even as more and more suspects get bail or are diverted to other programs, politicians and do-gooders run from the hard work of progress. They throw up their hands and want to throw open the gates.

The plan to close it and open other facilities, carrying a whopping price tag of $9 billion, got planning commission approval last month. The City Council, after one public hearing, is scheduled to vote Oct. 17.

To help guarantee passage in the far-left council, one member is pushing an inmates’ bill of rights, which includes giving inmates the right to decorate their cells. The bill would also require the new jails to have air conditioning, natural light and internet.

This being New York, there already is talk of turning the prime Rikers spot into a giant commercial and residential development. Imagine the legal graft the Tammany pols will get while ordinary New Yorkers get more dangerous individuals on the streets.

All in the name of compassion, of course.

But wait, it gets worse. Proving that the loony left is getting loonier, there is now a movement afoot to close Rikers and not bother opening other facilities.

Called “No New Jails,” the group has the backing of Rep. Alexandria Ocasio-Cortez. Naturally.

Consistent with her goal of eliminating immigration police (ICE), Ocasio-Cortez seems to be operating under the assumption that all inmates are good and all jails are bad.

It is a newspaper staple to say “the future is uncertain.” That is no longer true in an era where every bad fringe idea instantly goes mainstream.

So the future is clear. Prepare to run for your life.

Kim Don Un?

I’m still puzzling over the part of President Trump’s tweet on pulling troops out of Syria where he vows that “I, in my great and unmatched wisdom,” will punish Turkey if it does anything wrong.

So far, I’m down to two options: Either Trump was joking, or he’s been reading too many letters and statements from Kim Jong Un.

Whistle China for NBA foul

Every now and then, it’s good to remind ourselves that America is fundamentally different from China. In case we forget, count on China to remind us.

It’s doing so now with its explosive reaction to a tweet from the general manager of the NBA’s Houston Rockets expressing support for protests in Hong Kong. Even though the GM, Daryl Morey, apologized and deleted the message, and the NBA itself groveled, the backlash has been heated and expensive.

China canceled agreements and called off plans to televise games. Suffice it to say Chinese fans were not consulted.

NBA Commissioner Adam Silver was late to grasp the significance of the reaction, but is now acting with more sense.

“The long-held values of the NBA are to support the freedom of expression and certainly freedom of expression by members of the NBA community,” Silver said Tuesday, pointedly including Morey as enjoying “that right.”

Having seen the light, perhaps Silver will also recognize the rights his fellow Americans enjoy when they disagree with the NBA. Recall he refused to hold the league’s All-Star game in Charlotte until North Carolina changed a bill on bathroom genders.

By that standard, the NBA wouldn’t set foot in China.

Lifelong Dem: Why I went, ‘Hill, no!’

Reader David Rocco shares his response to the many Trump-haters around him. He writes: “I am a life-long Democrat who is surrounded by friends and family asking me, ‘What happened to you?’

“My response is Hillary. But more important, never again should there be a situation where a handful of powerful people try to destroy a political candidate and hijack an election. I hope Attorney General Bill Barr punishes all the individuals who took part in this travesty.”