We meet him on the march in 1834, adroitly reversing the grim royalist position in the First Carlist War — a liberal-vs.-conservative civil war that also mapped onto ethnicity, geography, and royal succession.
On this occasion, he overwhelmed a contingent of liberals and Basques fighting for the child-queen Isabella II. The survivors were taken prisoner and (despite objections from some of Zumalacárregui’s underlings) given a fusillade the next day in the neighboring town of Heredia.
This pithy diary entry from a Carlist officer comes from the incident’s Spanish Wikipedia page:
Día 17. Permanecimos en Heredia donde se fusilaron 118 peseteros. (“Day 17: We remained in Heredia, where we shot 118 Chapelgorris.”)
The Fusilamientos de Heredia — still notorious to this day — were distinguished by their number, but they were hardly unique. Both sides in the civil war unapologetically carried out summary executions of prisoners they had no resources to detain and did not care to turn loose. (And in the more everyday interests of sowing terror, or avenging the last time the other guys sowed terror.)
An English peer eventually brokered the Lord Eliot Convention, an arrangement by which both Carlists and Cristinos agreed to stop slaughtering prisoners and exchange them so that they could properly slaughter one another on the battlefield instead.