In his young adulthood, Diego loyally served his uncle, the King Ferdinand III of Castile.

Diego, his cousin, continued to serve him in his same position, but their relationship was incredibly strained.

Finally, Diego exercised his right to refuse obedience to the king (a law recognized in Middle Age legislation to feudal lords) and began searching for another qualified candidate to become King of Navarre.

He was killed in a bathing tub filled with boiling water under the pretext that it would cure his rheumatism.

He was buried in the Monastery of Santa María la Real of Nájera where his father and family members were previously entombed.