[Halbarad]

Man I've been gone a long time- I don't recognise anyone. Having said that, I think Frodo is an ideal picture of how pure a person can be. Indeed, Saruman says right at the end "You have grown indeed halfling." As others have said, Frodo would not bring himself to kill, but that does not in any way make him weak. His body may have been weakened but not his soul. I say this for many reasons:
-Frodo learned from Gandalf in Moria, with the Gollum debate about dealing out death in the name of justice. He learned it from one of the wisest beings in Middle earth. The Valar did not kill people such as Feanor, even though he did many terrible things, does that make them weak?
-Sam was stupid, plain and simple, but rustic people have their own wisdom. If Sam had borne the same burden , he would have succumbed, in my opinion- he did not know how to deal with it (we see Sam flustered whenever he gets responsibility, while Frodo remains calm throughout).
-Frodo killed in th LotR- remember the cave troll? He killed only when absolutely necessary. Remember it was he who kept the Shire hobbits from killing the ruffians in the scouring of the Shire, when it would have been tempting to punish them for ruining the Shire? He even wanted Grima to stay alive, not out of weakness but out of mercy- just as the Valar were merciful to Melkor in the beginning. In a similar situation Aragorn was merciful to Beregond, when he committed an offence punishable by death in spilling blood in the citadel. I argue that it is the same thing as Frodo's reluctance to kill.
It is tempting to call peacemakers weak, but as Joy pointed out, it takes a stronger person to stay the hand in mercy than it takes to strike with the hand.