Both claimed Cairns approached them in 2008 to fix matches.
McCullum told Radio New Zealand that the case became about him versus Cairns.
"I felt that I was the one on trial and I found that really hard to deal with," he said.
McCullum added he was not a person who held grudges but he could not forgive Cairns.
"The last thing I wanted to see was one of my former teammates rotting away in a cell - and I'm actually pleased he's not. But I can't forgive the other behaviour, the way that [he] effectively lobbied to put me in the gun publicly.
"I want nothing more to do with him."
McCullum said he felt he had to come forward with his allegations against Cairns.
"I didn't want to necessarily report it but when I found out that you're as liable for conviction as what the person who approached you is ... then it became apparent that I had to step forward," he told Radio New Zealand.
"Once people understand that and the fact I didn't want to rat on my mate, they will hopefully understand a little bit better."