They shot someone who was running and attempting to shoot them. It turns out the runner was an illegal alien who had 800 lbs of marijuana in his vehicle - and his own mother said he never sneaks in without being armed. It turns out that later he was caught smuggling more marijuana into the country, but the Bush administration gave him amnesty (again) in exchange for his perjury against our BP agents (who are US citizens).
Salo, your information is wrong. The Mexican was running away. He wasn't shooting at the agents at all. BearCub is correct.