What I have found will either save or destroy you. Perhaps hundreds of thousands of years. They own nothing which they have not stolen. I can barely make sense of their incessant rhetoric, except to know that you seem to be their Devil. Congratulations - you manage to make friends wherever you go and, apparently, places you haven't.
All indications are that the marine in Halo is a military cyborg similar to the one in Marathon.
It is generally assumed that the Marathon marine has Jjaro implants. Hopeless encounters successfully won. A man long dead. Grafted to machines your builders did not understand. We might speculate further that since mankind have created the marine they have committed some form of sacrilege in the eyes of the Covenant and thus must be destroyed with religious fervour.