The Sinn Fein politician is now faced with fresh accusations that he is withholding information from the devastated families of IRA victims.
Michael Hand, the brother of Detective Frank Hand, who was shot by the IRA in a post office raid in Meath in 1984, said he believed the death forced his parents to an early grave.
The officer died in a hail of bullets while accompanying a cash delivery van to a post office in Drumree, Co Meath. His wife Breda was pregnant at the time of her husband's murder.
His brother Michael has today revealed the years of pain his family has suffered.
And he has accused the Ulster MP of "being involved" in his brother's murder and claimed that the idea of him being elected president was an "abhorrence".
"As far as I'm concerned, he has my family's blood on his hands. He was 25, he was five weeks married. He has a daughter now. He never saw her," he said. "It opens up everything, the fact that McGuinness has entered the race here. And the emotions stirred up has really upset a lot of my family as well.
"Both my parents are dead now but to my mind, it resulted in their early death. It broke their hearts and it caused difficulty in my family. There were seven of us, he was the middle brother. It was very traumatic."
Mr Hand said he accepted the work Mr McGuinness had done in relation to the peace process but that his family felt "insulted" that Sinn Fein never issued an apology.
"I accept that he was instrumental in bringing things about (with the peace process) but I think he has blood on his hands and I think he's an inappropriate candidate for the presidency of our country, of my country."
"We all know he was involved and to say that he never would have is ridiculous."
Mr Hand's emotional attack on the Deputy First Minister comes after a day after a dramatic confrontation with the son of a murdered soldier.
David Kelly, whose father Patrick was shot by the IRA in Co Leitrim in 1983, has vowed to launch a campaign against Mr McGuinness until he names his father's killers.
The 35-year-old private --the first soldier killed in active service in the Republic since the Civil War -- was murdered alongside Garda Gary Sheehan (23) as they were searching for kidnapped supermarket boss Don Tidey.
Sinn Fein said that the issues "had been addressed" by Mr McGuinness already and that it had no further comment to add.
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