The man who shot and killed Niecy Nash's brother 19 years ago has allegedly killed AGAIN -- this time murdering his wife ... and the comedian says she's appalled that another family has been ruined by this "heinous" killer.

Robert Lee Heard was arrested Saturday in Shreveport, LA for allegedly stabbing and killing his 35-year-old wife Demetra Doyle Heard.

In 1993 a 16-year-old Heard was convicted of shooting and killing Nash's brother, Michael Ensley, at a Reseda, CA high school. He was committed to a juvenile corrections facility until 2002.

Then in 2003, he was convicted of attempted first-degree murder and sent to prison ... but was released in 2011. That is until this weekend ... when police found his wife stabbed to death.

Nash tells TMZ ... "No family should have to endure the loss of a loved one at the hands of a previously convicted violent criminal." 

Nash added,"I am deeply saddened to hear that the man who murdered my brother, Michael Ensley, has been charged with murder again."

And Nash went on ... saying "I grieve for every family who has been victimized by this heinous individual.  My prayers and love go out to the family of Demetra Doyle Heard during this trying time."