And Sara Canning said the New IRA who robbed her of a planned marriage to Lyra were a “scourge” on their own communities in Derry.
Ms Canning said the young people who had killed the woman she was going to marry had been groomed and attracted into a life of violence.
“They are grooming young men and women,” she said in a powerful interview for Channel 4 News.
“They’re no better, they speak out about paedophile gangs, but they’re no better than paedophiles.
“They literally take young people who are disenfranchised at the best of times, who are living in poverty, who don’t see a future for themselves in Northern Ireland because there is literally very little here for them and they tell them that the way forward is a gun in their hand.
“The way forward is never a gun in your hand.”
Ms McKee was a bystander when she was shot in the head during rioting in Creggan on Thursday, April 18.
She was a promising young Belfast author aged 29.Speaking about the murder, Ms Canning told Channel 4 News: “It was so indiscriminate. There was no regard for human life… They took away an amazing person.
“I’ve lost the person I was supposed to spend the rest of my life with.”
She added: “We’d planned the rest of our lives together and in one split second some idiot with a gun took that away.”
Ms Canning described the statements issued by Saoradh and the New IRA after her death as “pathetic” and “mealy-mouthed apologies”.
“You can’t apologise for killing someone,” she said.
“You can’t call it an accidental shooting. You fire a gun at a crowd, it’s not an accidental shooting. You’re aiming to shoot someone.
“I mean, it’s ridiculous. They’re not defending anyone. They’re literally a scourge on their communities.
“They’re a scourge. They are a scourge.”
In a message to Northern Ireland leaders, Ms Canning said they needed to look beyond their red lines in an effort to restore powersharing at Stormont.
“We need to look beyond red lines at the minute,” she said.
“Same-sex marriage and an Irish language Act are important. They are important, don’t get me wrong, but what’s more important is having a working government who are doing what’s right for the people of Northern Ireland.
“Because we are, we’ve been left adrift.”
In the interview, Ms Canning recounted that at Ms McKee’s funeral she had accused Prime Minister Theresa May of a dereliction of duty to the North of Ireland.
“I basically told her (Mrs May) that she was massively derelict in her duties to Northern Ireland… to tell us that issues like gay marriage and abortion rights are a devolved matter is completely out of order,” she said.“We’re not a devolved state anymore. We’re completely unsuccessful. It has been shown that we can’t work together – so the British Government has to be our government. And they have to be our voice and they have to do better.”
Ms Canning said she told Irish premier Leo Varadkar that he needed to use his position as a world leader to facilitate talks aimed at resolving powersharing at Stormont.
“Karen Bradley offered her condolences and I accepted them graciously,” Ms Canning said.
“I wasn’t rude. But I did tell her that she was doing a pretty terrible job as the Secretary of State. Her lack of knowledge around Northern Ireland was appalling. She needed to go and educate herself.”
Ms Canning added that Ms Bradley had agreed with her.
“As a Derry woman, I had to say to her, to offer your condolences on Lyra when you have turned around and said that soldiers who indiscriminately opened fire in the Bogside on Bloody Sunday were doing their job in a dignified manner. I said they are no different to the thug that opened fire in Creggan on Holy Thursday and shot Lyra.”
Ms Canning also told reporter Alex Thompson that she was still trying to come to terms with Ms McKee’s death.
“I still expect to hear from her every morning,” she said. “I expect to wake up beside her, and I don’t.”
She said on the night Ms McKee died she had “gone into reporter mode”.
“Initially, I… Lyra was clumsy, and I thought she’d tripped over her own feet… and I looked down, and I knew straight away that she hadn’t tripped,” she said.
“And then I noticed that she had a head wound and I put my hand to it and I started screaming for help.”
Ms Canning and Ms McKee had been due to get engaged on Wednesday.
Discussing their relationship, she said Ms McKee had planned a really elaborate proposal at a lake house on the lake in Central Park.
“It was incredibly romantic, and she was such a thoughtful person,” she said.
“She put her heart and soul into everything for everybody. She was always making sure that people were OK and wanted to make everything as perfect as she could.”
Independent charity Crimestoppers has offered a reward of up to £10,000 leading to the arrest and successful conviction of Lyra’s dissident republican killers.