darth sidious was a jedi once
my speculation is as follows.
darth sidious used to be a jedi named sifa-dyas (or the earlier sido-dyas) in fact he was the head of the jedi council untill he presumably died about 10 years before AOTC. sifa-dyas was of course one of darth sidiouses many alter egos. this explains who deleted the kamino sysyet from the jedi archives. (yoda>"only a jedi could have ereased those" it also explains who crreated the army and why. darth sidious/senator palpatine-supreme chamsalor plapatine/sifa-dyas must have been a busy man.
i hope ep. 3 flashes back and shows what palps looked like when he was the head of the jedi order. i presume he looked super old like in ROTJ , after all he can make himself look younger than he really is (hence how the jedi didnt know that sifa-dyas and palps was the same dude).