Why would "Simon" kill everyone if they've given everyone a freeday? That's a valid order and T's should be expected to have a freeday until the next order from "Simon". If T's take a step then that is fine because the order was "freeday".
Think about this outside the context of "Simon Says". If I say "All Ts freeze", then say "All Ts have a freeday", of course Ts are going to unfreeze because they have a freeday now. A freeday does not mean "you must follow the previous orders until I tell you not to, at which point you can then have a freeday". A freeday means "do whatever you want as long as it's not rebelling".
Simon Says should only be used as a voluntary deathgame, not a forced deathgame. A forced deathgame must be map-made.
"If it seems unfair, I feel as if the t could go back to whatever the order originally was."
First of all, this is assuming this is a voluntary deathgame since forced deathgames must be map-made, and the map must do the killing. If you feel it's unfair, then no you can't just go back to whatever the order originally was because you signed up for the voluntary deathgame. Regardless of whether or not it's fair, you decided that "yeah, I'll take the risk of dying for some fun"; you should not be able to then go back on this decision after deciding the deathgame is not for you.
I noticed there isn't anything on voluntary deathgames in the rules ( @Dominic ), so I will assume that voluntary deathgames are purely a player-made construct and that anything that happens in a voluntary deathgame is accepted by the T once they've indicated consent. If it's unfair, that's on you for accepting a voluntary deathgame, so you should not be able to go back to the previous order.
Good questions though.