You have a twisted view of history.
The Mexican government members were abusing Texan Mexicans for quite a while before the Alamo. They got their ass handed to them, and America ended up with a new state some time later by the vote of the people.
You could get quite an argument from Mexicans on this. Among their beefs was Anglo Texans defiance of laws prohibiting slavery -- Mexico threatened military force over this-- and their refusal to be naturalized as Mexican citizens. Texans had to become Catholics, but it seems that rule was not enforced. But I imagine Texas's distance from Mexico's government, which tried to become more centralized and reneged on some commitments to settlers had a lot to do with the move for independence. The US paid back later on, with things like the "Greaser Act," theft of land, and the signs "No Dogs or Mexicans Allowed," etc.
FYI, Santa Anna, our villain, is also Mexico's, for his inept handling of Texas, and later, the Mexican war after which he conceded so much land to the US. The history is colorful and weird --- Mexico still has a warm spot for "Los Patricios," Irish immigrants who switched sides during the war. Many were hanged by their US commanders.