I thought it's supposed to prevent me from being instakilled?
nope. all sd attacks = insta crit no matter what.
SD gives a 10% bonus on the crit table which is half the bonus of better crits, having man of steel negates that but you still have the 20% from better crits they invariably will be using. Downgrading all crits against you is pretty important because it also protects against crippling and knockdown effects plus combined with bonehead and the critical reduction of helmets you can negate at least 30 on the crit table making their perks useless.