Update 12.54pm: 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, hopes the Sinn Féin member'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 Féin 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."
Update 11am: Sinn Féin's leader in the North, Michelle O'Neill, has responded to the news of Barry McElduff's resignation.
Her statement on Sinn Féin's website said: “Yesterday evening, Barry McElduff informed me of his intention to resign as Sinn Féin MP for West Tyrone.
“Barry is doing so as a consequence of the unintended hurt caused to the Kingsmill victims and their loved ones by his recent social media tweet.
“Barry recognises that this controversy and his continuing role in public office is compounding the distress to the victims of Kingsmill, and again offers his profound apology to those families and to the wider victim’s community.
“He has said that he does not want to be a barrier to reconciliation and I respect that decision.
“Barry has served Sinn Féin and been a formidable champion for the people of West Tyrone at local government, Assembly and Westminster level over the past 20 years and has done so with great commitment, energy and determination.
“For this I want to personally thank Barry and his family, Paula, Niamh, Blannid and Patrick.
“Over the coming weeks Sinn Féin will focus our full efforts on the restoration of the power-sharing institutions on the basis of equality, integrity and respect and fulfil the mandate we received from the electorate in two successive elections last year.”
Earlier: Sinn Féin MP Barry McElduff has announced his resignation this morning.
In a statement on the party website, the West Tyrone MP said: “It is with great sadness that, after more than 30 years as an active Sinn Féin member and public representative I am tendering my resignation as MP for West Tyrone.
“The reason I am doing so is because of the consequences of the Twitter video which has caused such controversy over the last week.
“But the deep and unnecessary hurt this video caused the families of the victims of Kingsmill is my greatest regret.
“I again offer my profound apology to those families and to the wider victims community.
“Had I been conscious of the connection to the terrible atrocity at Kingsmill I would certainly not have posted that tweet. I genuinely did not make that connection, not for a second did I make that connection in my mind.
“Kingsmill was wrong, unjustifiable and sectarian. It should never have happened.
“There was no intended reference to Kingsmill in my tweet. But I do accept that there are many people who do not believe this to be the case. I accept also that this view of what happened is deeply damaging to the reconciliation process that is so important to consolidating the peace process and to healing the pain and hurt of the past.
“I cannot undo the pain caused but I know that my continuing role as MP for West Tyrone will compound that sense of hurt and impede any reconciliation process.
“I wish to wholeheartedly thank my family and friends for their steadfast personal support during this difficult time, and the people of West Tyrone whom I have had the privilege to serve as their public representative for over 20 years. I have a deep gra for my native county and its people.
“I am an Irish republican and believe whole heartedly in the reunification of our country and an agreed Ireland in which we heal the wounds of the past together.
“Reconciliation is essential, but that message is not being heard at this time.
“I do not wish to be a barrier to reconciliation and healing and in that spirit I again offer my sincere apologies to the survivors and families of those murdered at Kingsmill.”
Last week, Sinn Féin suspended Mr McElduff from all party activity for three months over the affair.