You really believe hidan can kill himself? I don't think Jashin would allow that he let him become immortal for a reasonlook hidan is pretty strong but let's not get over ourselves man...hidan may not even know he is in the genjutsu already and if he's to be tricked into believing he's in the circle while he is not then he'll just defeat himself or atleast render himself weakened..
hidan is dangerous to attack head on and ninjutsu may not hurt him much but let's be honest here he isn't the sharpest knife in the drawer