One of the most excruciating, heartbreaking stories you will ever hear ... especially considering it should have never happened.

Mixed martial arts fighter Marcus Kowal pulled the plug on the life-support machine hooked up to his 15-month-old son, Liam, on Tuesday night, three days after he was struck by a drunk driver.

According to the Daily Mail, baby Liam was being walked in his stroller by his aunt in California on Saturday, when a 72-year-old woman crashed her vehicle into them, leaving the infant brain dead a day later.

What's worse is the elderly woman, Donna Marie Higgins, fled the scene in a hit-and-run before she was caught and arrested on felony drunk-driving charges.

Just before turning off life-support, Kowal posted this heartbreaking photo of him holding his son's hand.

Hours later, he took to his same Instagram account to plead with people to not drink and drive.

Just heartbreaking.

After Liam passed, Kowal and the child's mother, Mishel Eder, announced that they will donate the infant's organs "so that his little heart can make another child live and another set of parents not have to go through what we have to."

Higgins's court date isn't immediately known.

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