Sorry for the circumstances. Predators will get birds flying around in the pen after dark and reach through the wire and kill them. I have had birds killed this way by both Great Horned Owls and raccoons. I doubt that your chickens killed the guinea fowl.
My problem with owls was with ring neck pheasants. My pens were 8 ' tall but were covered with a large mesh wire. The owls would get the pheasants flying in the dark and grab their heads through the wire. I replaced with smaller mesh and solved that problem - then had trouble with snow in the winter.