He did.
He warned Hashirama (The CURRENT HOKAGE) and his Clan. What more should he have done?
He warns Hashirama and his clan. He stays alert throughout his life, but stays peaceful in the village. Tobirama becomes hokage. Tobirama tries to subjugate the uchiha.
Madara and uchiha try to revolt. Revolt doesnt work, so they rebel, and try to stop tobirama, while avoiding destroying the village.
They stop tobirama and the people with him who tried to subjugate them, then they go back to being peaceful, and help rebuild the damage they caused.
Saying madaras choice of attacking the entire village to stop a POSSIBLE outcome is the right decision, is bull crap.
Its like saying its fine for me to shoot someone who is in the alley with me in a sketchy part of town. Sure it might be likely that they will attack me. But until they actually begin attacking me, or approaching me threateningly, I should not shoot.
Using that logic, It was the right thing to do for Zimmerman to shoot Trayvon Martin. Because trayvon might have robbed one of his neighbors. (and his neighbors had been robbed several times over the years).
It is FINE to take preemptive action to prevent those things from happening. It would be fine for the uchiha to have their defenses up, so they are not subjugated. It is fine for the person in the alley to have a weapon on them in case someone attacks. It is fine for zimmerman to be alert and ready if trayvon tried something(and the reverse as well).
It is a whole other thing for someone to act on it, before it happens.
You dont know what the future holds, so you should not make serious actions based on just one of the possibilities. You dont just go arrest kids in school because they might rob a bank in the future. Or rape your sister. You keep your eyes open in case they do so you can stop them, and then punish them. But you do not attack them or imprison them for a crime they have not commited.
Disclaimer. This post is NOT a discussion about the zimmerman case. I used it, because it is a example of something that would be fine and moral with your ideals, but that you are probably opposed to. I am not saying either person was innocent, or guilty. I am just using it as a example of killing someone because of their possible future actions is wrong.
Finally though, what happened to the uchiha is not madaras fault nor tobiramas fault. It is the uchihas fault, and danzo/hiruzens fault.
The uchiha did not have to plan rebellion, and the leaders of the village did not have to kill the uchiha for actions they had not yet fully committed.
It is madara and tobiramas fault that the uchiha were placed in the somewhat oppressive situation they were in, but not their fault for the rebellion in planning, or the massacre.