Is this legal for me to do?

Question

To sit on the border of texas and shoot people who are about to cross into the United States illegally?

Answer 1

NO, MURDER IN THE  FIRST DEGREE AS WELL AS PREMEDITATED MURDER IS FROWNED UPON IN MOST IF NOT ALL OF THE WORLD. HERE IS JUST A SUGGESTION NOW I DON'T WANT TO ANGER YOU, BUT HAVE YOU EVER LOOKED INTO STARTING A HOBBY.

Answer 2

It's a shame you can't

Answer 3

You can be prosecuted for capital murder in Texas and probably also in Mexico.

I assume this was supposed to be a jurisdiction question.  You committed the act (firing the bullet that killed someone) in Texas.    Texas will have no problem prosecuting you.