You keep using the word "prompt," but that doesn't appear anywhere in the treaty. http://www.tsl.state.tx.us/treasures/republic/velasco-private-2.html
"Art. 5th The prompt return of Genl. Santa Anna to Vera Cruz being indispensable for the purpose of effecting his solemn engagements..."
Alright, let me see if I've got this straight. Santa Anna was already facing revolts at home by whole regions of the country that found his seizure of power to be illegitimate. He is captured and, while a prisoner of the Texans, many of whom are calling for his head, he signs a treaty, but with the condition that he has to get back to Mexico ASAP to ensure that he's still in power in order to make the treaty work ("being indispensable for the purpose of effecting his solemn engagements..."). The Texans, however, don't let him go for another 8 months, he does lose power and therefore is unable to "effect his solemn agreements").
And then you're angry that the Mexicans, who were trying to overthrow Santa Anna when the Texans captured him, repudiate a treaty that he signed giving away a good chunk of their territory in order to save his own skin?
Is that about the size of it?