I don't really lump Hagel in with Lugar, whom I disagree with on occasion but nevertheless respect.
Hagel may be motivated by jealousy, as you say, or he may have a sort of maverick or knight on a white horse complex -- a windmill tilter. Usually such traits are more suited to Demos, so I think it would be an interesting project to really explore the roots of his affiliation with the Republican Party. Interesting, that is, if I gave a damn about the guy, which I don't.
LOL!! That sums up my feelings as well.
I'm betting a little bit on bitterness. Hagel was one of the top choices for the Vice President slot under Bush. In fact he even bought some of the domain names for a Bush/Hagel ticket. Rove liked him and wanted him, since Hagel had 3 solids, he was conservative, a vietnam veteran, and known for being very independent, but never the less solid. Dick Cheney even interviewed him (Cheney was head of the committee to pick the VP for Bush, him and his committee's conclusion was that he was the best guy).
Hagel was mad, Rove fought against Cheney being named tooth and nail, lost that battle and moved on. Hagel probably still holds a grudge over it.