I will fight for Force Sensitive Fives with my life!!! That’s just the only explanation I can think of. Yes, he was an EXCELLENT soldier and all his training made him the best, but during many episodes he’s in focus as a main character, he seems to have “a feeling” when bad things are going to happen. (And that thing I point out a while ago about him moving his helmet antenna without touching it??? Yes!) So yes, force sensitive Fives please!!
And yes! He was the person who was the closest to prevent order 66 and Palpatine’s rise. I’m sure that Rex would have listened to him if he had given a better explanation (like talking clearer, being calmer), but Nala Se had drugged him and that plus Palpatine confessing him the truth, made him very confused and scared, that’s the Fives we see in his final scenes. He went from a brave, composed, expert, trustable trooper, with a clear identity (he had this since the beginning “It’s FIVES! The name is FIVES!”, we hear him say just as a cadet), to an scared, “crazy” person who was talking about conspiracies, who , in the eyes of basically everybody else but us, was infected with the same virus as Tup and had almost killed the Supreme Chancellor. And the most painful part is, for somebody who tried so hard to be considered as an individual, as “not another number”, he died like a nobody, in a blank armor, not in his ARC trooper armor, without his hair (something that defined him), and loosing all the credit he had as a trooper. He had always been worried about protecting his brothers and lost everything he was trying to do it.
And yes again!, he could have saved millions if somebody had listened to him. By telling everybody about the chips and that Palpatine was behind it, the Jedi would have investigated more and hopefully, they would have realized that Palpatine was the Sith Lord. The Jedi purge would have never happened, the Empire would have never risen, clones would have been, at least, freed from the thing that basically turned them into mindless droids. So many lives could have been saved, if only they would have listened to Fives.
Of course we had Kix, who was suspicious about all this and went to investigate on his own, but he was kidnapped and never got to tell the truth. And by the time Rex knew, it was too late.(I really hope I’m making a sense with this! It’s the first time I’m trying to write my thoughts, at least in a “clear” way)