So, if you have added nagios to the root group, and changed the owner of the files in the libexec directory to root, then how can we help you? I'm not sure, but it looks like you have decided that the documentation of nagios is useless. I see nowhere in the docs where you are instructed to do as you have done.
I've said this a dozen times so far. EVERY time I install nagios, I follow it step by step in the docs, never deviating. They are complete, precise, and to the letter. There are NO errors in the nagios docs that I have found.
If you intend to deviate from the docs, then you have apparrently decided to go about this on your own. Please don't, it works great the way the docs instruct. If you ever decide to follow the docs, then please post your troubles, if you have any.