So I am not 100% you mean 'mercy killing' which in general means killing to spare from greater suffering - a mortally wounded comrade who will suffer terrible pain for days before death is killed out of mercy to save them suffering. I think, if I read your wider context correctly you are referring to the treatment of wounded or captured foes and the value of human life in wartime throughout history. If I have mis understood that, please forgive me, but I will try and answer what I think you are wanting to know through the context of my own period of study - the extensive warfare in western Europe during the seventeenth century.

First some wider context to how society viewed life - it wasn't so much that they viewed it as cheap, just that death was so commonplace and all around, that it was an excepted and expected thing to happen. Secondly, for the overwhelming majority of the populations, life was so unbelievably back breakingly hard, that you had to focus on the good of those closest to you - there was no surplus to spare for others, and although the bible preached mercy and kindness, and sacrifice, this was more the ideal than the reality - why give a penny to a starving stranger when you might need that tomorrow to feed your child? I say this merely to try and frame their views. Obviously the wealthy and more secure were better off, and could literally afford more compassion for others. In fact, in England during this period, the entirety of poor-relief and assistance for those in need was drawn from wealthier local people - there was not state aid or support.

Right so onto wartime. Again, we must realise that sickness, mortality, absenteeism, etc. meant that no army was ever "fully manned". Contemporary soldier and author Francis Markham commented that 'war is a known enemy, and who knows not that sickness, mortality, slaughter, ill diet and lodging, hunger, cold and surfeits doe so attend upon Armies, that by them companies are exceedingly weakened and mad less, so that he which mustered one hundred men if he bring three score and ten able men into the field to fight, is oft held for a strong company."^(1) 70/100 men - so a 'strong' company was operating at 70%, most were far less. Mortality (and desertion) was so high in armies that the historian David Parrott describes recruitment for early modern armies as the intermittent tap attempting to keep a bath filled with water (as an army with its men) when the bath’s plug is left out.^(2)

This casualness about mortality is a reflection of the reality and made its way into military reports. Contemporary battles were rarely clear on the numbers of men killed (also because a defeated army usually ran away or withdrew rather than fought to the death). The knowledge that even simple wounds could kill, and fleeing soldiers might desert after a battle, meant that estimating an enemy's losses was guesswork at best. The official Royalist report following the battle of Edgehill in 1642 serves as an example; it is very precise on the number of colours taken but somewhat vague on the casualties "The Rebels in this Battell lost above 70 colours of Cornets and Ensigns; we 16 Ensigns, but not one Cornet; but our Horse relieved not only the Standard, but divers of our Ensigns. For the slain on both sides, the Number is uncertain; yet it is most certain that we killed five for one. It is true, that their Chief Officers having fleeter horses than ours, not so many of their Foot, as ours, were slain and taken Prisoners, to our knowledge as yet; but we lost no Officer of Horse excepting the Lord Aubigny."^(3)

Ian Atherton’s work on battlefield casualties shows that rank-and-file soldiers killed in battle were buried swiftly, usually on or near the battlefield, and often in convenient mass graves with little effort to officially record their numbers or identity.^(4)

There is much more that could be said about treatment of prisoners, wounded, discharged soldiers, sieges and assaults, logistical necessity, civilians in war, etc. However I hesitate to go into more detail as I am not sure that is the drive of your question. Fundamentally though, the modern view on the value of human life is just that, very modern. Certainly in the period of my study, human life, at least of those unknown to you, was valued very cheaply, but that was a society-wide issue, not solely a military one. I would suggest it is more a reflection of our, in comparison, quite excellent standard of living in comparison to our forebears.

If you need or want more clarification or if I in fact read your question wrong - please feel free to ask.

References:

1. Markham, G., The Second Part of the Soldiers Grammar: Or a Schoole for Young Soldiers … For the Training, and Exercising of the Trayned Band, Whether They be Foote or Horse (London, 1627), pp. 47–48
2. Parrott, D., Richelieu’s Army: War, Government and Society in France, 1624–42 (Cambridge: Cambridge University Press, 2008), p. 178
3. A Relation of the Battel fought between Keynton and Edgehill, by His Majesty’s Army and That of the Rebels; Printed at His Majesty’s Command at Oxford by Leonard Lichfield, Printer to the University, 1642 (Oxford: 1642)
4. Atherton, I., ‘Battlefield, Burials and the English Civil Wars’ in Appleby, D.J., and Hopper, A. (eds.), Battle-Scarred: Mortality, Medical Care and Military Welfare in the British Civil Wars (Manchester: Manchester University Press, 2018), pp. 23–39