Francesca O'Brien, who was selected in the South Wales constituency of Gower, apologised on Sunday for the series of inflammatory remarks on social media.
Labour said Boris Johnson must remove her as a candidate over the "absolutely disgusting" since-deleted remarks from January 2014, which were reported by the Guardian.
Of the Channel 4 programme highlighting the lives of welfare recipients, Ms O'Brien wrote: "Benefit Street..anyone else watching this?? Wow, these people are unreal!!!"
Replying to a friend's comment, she added: "My blood is boiling, these people need putting down."
The Conservative Party did not appear to be moving to axe Ms O'Brien from the race ahead of the December 12 General Election.
In a statement, Ms O'Brien said: "These comments were made off the cuff, a number of years ago.
"However, I accept that my use of language was unacceptable and I would like to apologise for any upset I have caused."
Labour's Tonia Antoniazzi won the seat in 2017 with 22,727 votes, 3,269 ahead of the Conservative candidate.
Shadow education secretary Angela Rayner was critical of Ms O'Brien's remarks.
"This is absolutely disgusting and reveals the cruelty at the heart of the Tories' benefits cuts.
"Universal Credit was deliberately designed to punish people who are out of work," the Labour MP said.
"Many of us have relied on social security support at some point in our lives and it is nothing to be ashamed of.
"This person is not fit to be an MP and Boris Johnson should remove her as a candidate."