They claimed he did represent a threat to them, which is why a weapon was used.
If this were in rural Midwestern America, and the person shot an American citizen, we'd all be up in arms about jackbooted-thugs. But because the victim was an illegal alien, we want him shot like a dog?
The "victim" was an "unlicensed pharmacist" i.e., a drug dealer who was smuggling drugs into this country. Since he escaped, we have no way of knowing if he was armed or not or what action he took when confronted by the BP. Perhaps he was armed or made a gesture or action that could have been interpreted that way. You seem so willing to believe a drug smuggler's version of events than law enforcement's.
A jury weighed the credibilities, and decided they believed the smuggler rather than the officers who tried to cover up their crime. It's called falsus in uno.