[Farael]

I would not call what Sauron did to Denethor and Saruman to Theoden "attempt at assasination" as a matter of fact JeffF hit the nail in the head, they were controling them

Now, being controled by the bad guys clearly has some negative side-effects on people's health, but both Sauron and Saruman were better served by their targets survival, which would have allowed to weaken the good guys further than by just killing the leader and having the next in line step up, as demoralizing as that may have been.

Of course, as far as assassinating their characters... well, that's more like it