Homeless man, Toby Sawyer was 27 when he knifed his friend Gary Moultrie, 38, to death in a row over white emulsion paint.
Mr Moultrie had daubed his friend with the paint prompting a violent response.
The 46-year-old killer received a five-year jail term at the Old Bailey in 2000 after the attack.
Mr Moultrie’s sister Margaret Austen, 71, told The Sun: “I’m not surprised the rail workers wanted to get rid of him because as far as we’re concerned he is dangerous to other people.
“He stabbed Gary through the heart and he died from massive blood loss before they could get him to hospital.
“That man was only jailed for five years, it was devastating, not just to lose Gary but also because the man who killed him served so little time in jail.”
Sawyer has been living on the streets since his release from prison according to reports.
Train staff – with one wearing a Southern Rail fleece and another wearing a Southern Rail badge – were filmed tipping a bucket of water around the homeless man as he lay outside Sutton station.
Southern Rail’s parent company GTR said they were “aware” of the video and are taking it “very seriously”.
Homeless charities slammed the scenes which they described as “heartbreaking” and “terrible”.
Witnesses also described the video – which has been shared on Facebook thousands of times – as “disgusting” after the homeless man gets left soaked in soapy water.
The clip begins showing Sawyer with his belongings laying beneath an information sign outside the station in Sutton.
Water filled with suds can been seen on the pavement, before one of the workers dumps another bucket load near him offscreen – with the water flowing onto the man.
Sawyer fidgets and shares a few words with the Southern worker, who then dumps another load of water next to him.
People can be seen walking past, and the woman behind the camera says “absolutely disgusting behaviour from another human being” and “he is throwing water all over the man”.
A Govia Thameslink Railway spokesperson said: “We are taking this very seriously and are in the process of conducting an internal investigation in regards to the online video.”