I agree with you that the hitmen were supposed to be double crossed. But there were two hitmen and as you say that would take two men to silent them. So Fredo was indeed involved and he did it to cover up the mess he made. He had no other choice to listen to the third person came with Roth's package to kill Mike.
And don't forget they came with Fredo, stayed at Fredo's and were told to hide at Fredo's.
If you'd like to see the rest of my statement please refer to the old thread we were discussing it at "A GF II question that's been killing me"
Thank you.