HK-31 was a defective model of HK-47. While his later models were orange and black, HK-31 was bright silver. It was unknown if he was killed by HK-47, although one droid that looked exactly like him was spotted on M4-78.
Smart answer: This unit was built on M4-78 and was designed to kill all people that master demand to be killed.
—HK-31 to Jedi Exile when asked where he was built
It was unknown which date that HK-31 was created, but it was known that Goto on M4-78 created him. M4-78 was under control by the Droid Empire at the time. It is also known that he was the first droid in what was to be called the HK-30-60 Droid Line. HK-31 was almost like HK-47 and the units that followed after; he called all humans meat-bags and had a bloodthirsty attitude. He was well known for his use of many different weapons and was known to kill many people on site for trying to talk to him. Around this time, Goto deactivated the droid and sold it to an unknown buyer. However, this event set in motion in what was to come.
Journey with the Jedi ExileEdit
It is unknown when the Jedi Exile purchased the droid although the previous owner of him was never heard from again. With the Jedi Exile, the droid traveled to many planets including Telos, Korriban and Dantioone. Eventually during the Final Battle at Malachor V he was destroyed by rocket fire.