Because whites for so long have reflected on the Mexicans this image of worthlessness and inferiority, for a while Mexicans internalized it and created their self-identity from that which they saw of themselves in the eyes of white people. They saw dirty, lazy, good for nothing people that didn't deserve fair treatment. The result then is a confused Mexican American in search of some self worth. All any body wants is to be treated fairly and that is all the vendidos want. To them they best way to do it is wipe out the reflected image whites have put on them, and through it all they have to remain human.
Sacrifices have to be made for la causa, but it is for the best.