Regardless of whether you've heard it or not,
"In 1829, Mexico abolished slavery, but it granted an exception until 1830 to Texas. That year, Mexico made the importation of enslaved people illegal".
The Alamo battle was in 1836, so, what was at stake was the ability for large agricultural and ranching operations in that part of Texas to either have slave labor or to have to have wage labor.
America won, so the enslaved workers lost. No one's glossing over the ruthlessness of various Mexican generals but that's not why they fought at the Alamo.