The highest court in Massachusetts on Tuesday overturned a first-degree murder conviction against a Boston man who stabbed his girlfriend to death, finding that jurors were given faulty instructions on how they could consider his use of alcohol the night of the killing.



The Supreme Judicial Court vacated the conviction of Mario Gonzalez and sent the case back to a lower court so prosecutors can either reduce his conviction to second-degree murder or put him on trial again on a first-degree murder charge.



In its ruling, the high court said jurors should have been told they could consider Gonzalez's consumption of alcohol when deciding whether he acted in a cruel or atrocious manner when he repeatedly stabbed his girlfriend, Luz Forty, after celebrating Valentine's Day in 2009.



Gonzalez was convicted of stabbing Forty multiple times during the early morning hours of Feb. 15 after they returned to their apartment from a local bar, where they had both been drinking and celebrating Valentine's Day with Forty's mother.



Gonzalez called 911 at about 3:15 a.m. and said someone had come into the apartment and stabbed Forty. Prosecutors said he later told police Forty had attacked him with a knife, which he said he twisted and used to stab her.



Forty was stabbed six times in her left shoulder, once in her right shoulder, and once in her chest. She died hours later.



The trial judge, Geraldine Hines, told jurors they should consider evidence relevant to Gonzalez's intent, including the effect of drug or alcohol impairment, but did not instruct the jury that they could consider his alcohol consumption in determining whether he committed the killing with extreme atrocity of cruelty. Hines began serving on the Supreme Judicial Court last month, but did not participate in the court's deliberations in the Gonzalez case.



The high court found that the error could have affected the jury's verdict.



"The judge did not instruct the jury that they could consider any credible evidence of the defendant's consumption of alcohol in determining whether the defendant committed the killing with extreme atrocity or cruelty, an instruction that in substance is required where there is evidence that the defendant was under the influence of alcohol at the time of the killing," Chief Justice Ralph Gants wrote in the unanimous ruling.



Jake Wark, a spokesman for Suffolk District Attorney Dan Conley, said prosecutors are reviewing the case to determine their next step.



"The facts of this case warranted a first-degree conviction," Wark said.



If prosecutors reduce the conviction to second-degree murder, Gonzalez, now 35, would become eligible for parole after serving 15 years. A first-degree murder conviction carries a mandatory life sentence without the possibility of parole.