One of the cartels had demanded that Garza give his ranch to them. He flatly refused. He gave his workers the day off, and fortified his home. When armed men surrounded his house the next day, he was prepared. He fired, killing four and wounding two before he was killed himself, an excellent performance against superior numbers and firepower.
Garza's death illustrates the dilemma of an armed citizenry. In a place like Northern Mexico where the cartels are powerful, armed resistance can be expected to be rewarded with death. Or worse, and more effective, they will just kill someone you care about who is an easier target. Garza apparently decided that he had nothing to lose that wasn't worth it.