The sole survivor of the Kingsmill massacre said he has been "left hanging by a thread" as a result of the controversy surrounding Barry McElduff.
Alan Black, who was shot 18 times and left for dead alongside the lifeless bodies of his 10 friends, said he hopes the Sinn Fein MP's resignation may help, in part, to heal his distress.
He said: "This past week has been truly awful for me. I am just hanging by a thread.
"But I am glad he has done the right thing."
Mr Black previously described the Twitter video, in which Mr McElduff posed with a Kingsmill-branded loaf on his head on the anniversary of the IRA gun attack, as depraved and designed to hurt.
He said the fall-out had forced him to re-live the trauma of that harrowing night in January 1976.
"I am going to have to take time now to heal," he said.
"I only got involved because of the hurt and disrespect shown to my friends who died at Kingsmill but this whole thing has taken a heavy toll."
The resignation came just hours after Mr Black gave a powerful RTE radio interview in which he accused the West Tyrone representative of celebrating the deaths.
"I did a radio show at the weekend and that was the last straw," added Mr Black.
"I am going to have to go now and lead a quiet life for a while."
Colin Worton, whose brother Kenneth was also among the victims, said the politician should have stepped down sooner.
"He should have gone. It took him a week and I'd like to know what happened in the last few hours to make him resign," he said.
Mr Worton, who revealed his 90-year-old mother has received abuse in connection with the massacre, also called for a change of attitude among some republicans.
"It is welcome news that McElduff has gone but it is still only a small step," he added.
"Sinn Fein need to do an awful lot more to change the mindset around glorifying terrorism.
"This past week has been very difficult for us.
"It is always difficult around the anniversary but this year it has lasted and lingered longer. It has been day and daily pressure.
"My elderly mother is also feeling under pressure. It does bring it all back."
Meanwhile, Mr Worton said he did not accept the apology and demanded that action also be taken against those who retweeted the video.
He said: "Barry McElduff knew what he was doing. He just did not think there would be so much reaction."