Sydney siege gunman Man Haron Monis should have "rotted in jail" and never been free to commit atrocities, said former in-laws of the hostage taker.

In an outpouring of angry messages on social media, family members of Monis's now-dead wife, Noleen Hayson Pal, described him as a "f****** sick animal".

Pal was married to the Iranian Islamist, but was brutally murdered in 2013. Monis's new partner was charged with her murder and he was charged with being an accessory to it.

His wife's remains were found in a Sydney stairwell in April 2013. She had been repeatedly stabbed and her body then set on fire.

Monis's wife had accused him of beating her during their marriage, and of brainwashing their children with Islamist propaganda.

Monis was described by his own lawyer during the trial as a "damaged goods individual".

The relatives of Monis's late wife also questioned how the man could have been judged not to be a threat to the public: "Why in the first place they let him free on the Sydney streets. They should have let him rot in the jail – he's a f****** sick animal."

Pal's god brother shared how he felt grimly satisfied that Monis was killed by armed response units at the end of the siege.

He wrote: "The systems are f***** up but I'm still happy he died. F****** lucky they got you before we did. Now rest in hell f****** a******."

Pal's godfather told NBC: "They should have put him away and thrown away the key."