He owned a construction company and served as the leader of the Movimiento Nacional in Guipúzcoa, as well as city councilor in the municipality of Azkoitia during the final years of Francoist Spain.
Arenas had just bid a friend farewell and had nearly arrived home when a group of men in a Chrysler or Renault opened fire.
On the night of 31 December 1984 he, along with his wife and brother-in-law came to the city, and he spent the evening with several lifelong friends, including the mayor of Azcoitia, Román Sodupe and José María Sudupe, brother of the president of Executive Committee of the Partido Nacionalista Vasco.
As soon as they saw him leave, "Kurika" and "Iñaki de Lemona" exited the car, approached him from behind, and shot him twice in the head, and once in the chest, killing him instantly.
For their role in the murder, Ignacio Bilbao Beascoechea and Miguel Angel Gil Cervera were later sentenced to 39 years in prison, and were also ordered to pay 50,000,000 pesetas in compensation to his widow.