A schizophrenic Manhattan man who fatally plunged a kitchen knife into his wealthy philanthropist mother as she slept is set to reap an unexpected reward — a $3.2 million inheritance.

Jonathan Schwartz, 46, was committed to an upstate psychiatric hospital in 2014 after a jury found him not guilty by reason of insanity for the 2011 murder of Barbara Weiden Schwartz Fischler.

The madman’s father, Steven Schwartz, who was divorced from his wife, sued his son in 2015 to block him from his share of his mother’s $6.4 million estate.

Tragedy has trailed the Schwartz clan since Barbara’s death.

The ex-couple’s only other child, Kenneth, killed himself six months after learning that his stepdad had squandered millions of the family’s fortune on risky short-sales.

Because Barbara died without a will, her spouse and children split her estate under New York law. But a prenuptial agreement bars the second husband, Burton Fischler, from any interest in his late wife’s fortune, according to court papers.

Kenneth’s share passes on to his biological dad, Steven, who is the sole heir in the dead son’s will.

In reviewing Steven’s suit, Manhattan Supreme Court Judge Geoffrey Wright noted the obvious.

The suit “was commenced to declare Jonathan Schwartz ineligible to inherit from his mother by virtue of her murder at his hands,” he writes in his ruling.

But he arrived at what Steven’s attorney said was a “wrong” conclusion.

Wright found that because Jonathan didn’t understand the “nature or consequences of his conduct” or “that such conduct was wrong” he should still be able to collect his inheritance.

Wright also awarded Jonathan’s lawyer $14,000 in fees.

Steven’s attorney, Linda Genero Sklaren, has appealed the ruling arguing that Jonathan must pay financially, if not criminally, for his wrongful acts.

She added that the Surrogate’s Court judge overseeing Barbara’s estate is the proper jurist to rule on the issue.

Michael Kutzin, a court-appointed attorney for Jonathan, said if Jonathan inherits the $3.2 million it will be put in a trust and used to pay for additional care beyond what is currently covered by Medicaid.

Legal expert Arthur Schwartz, who is not related to the parties and is not involved in the case, predicted that Judge Wright’s ruling will be overturned.

“Uniformly, in New York, a murderer cannot benefit from the killing, in any way,” Schwartz said.