If Shakespeare be for you, who can stand against you?

Believing the French were attacking his trains and free the prisoners for an attack on his rear, Henry V ordered the killing of the captured French.

“When this dolorous decree and pitifull proclamation was pronounced, pitie it was to see how some Frenchmen were suddenlie sticked with daggers, some were brained with pollaxes, some slaine with malls, other had their throats cut, and some their bellies panched, so that in effect, hauing respect to the great number, few prisoners were saued.” (Holinshed, Chronicles, p. 554) –

http://blog.oup.com/2015/10/shakespeare-agincourt-historical-revisionism/#sthash.60KuDWNI.dpuf

There must have been some justification. Shakespeare provides one.

FLUELLEN: Kill the poys and the luggage! ‘Tis expressly against the law of arms. ‘Tis as arrant a piece of knavery, mark you now, as can be offert. In your conscience now, is it not?

GOWER: ‘Tis certain there’s not a boy left alive. And the cowardly rascals that ran from the battle ha’ done this slaughter. Besides, they have burned and carried away all that was in the King’s tent; wherefore the King most worthily hath caused every soldier to cut his prisoner’s throat. O ’tis a gallant king. (Henry V, 4.7.1-10)

Same link.