Are any armies merciful? Is that the role of an army?
Yes. See Geneva conventions below.
In regards to the OP's question and a more general formulation of it, as in 'Are armies merciful?' - it isn't in the nature of such. The purpose of an army in battle is to defeat its opponents and that is best and most effectively done by killing, maiming, wounding those opponents. I would suggest that the best hope would be that an army be 'civilized'.
The Geneva Convention (to which the US is not a signatory) was and is a laudable attempt to achieve civility in war and, were it adhered to in all respects, might achieve that end. Regretably, its very provisions, in some regards, contribute to its disregard by warring nations.
An example: Non-combatants are defined in the GC to include medical personnel and signatories are abjured to refrain from harming them (picture, if you will, the chivalrous acts, sometimes referenced in reports of battles when a truce was declared to allow collection and withdrawal of the wounded from the battlefield). To assist armies in meeting this standard, the GC looks for medical personnel to be readily identifiable by armbands or markings on helmets of an appropriate symbol (red cross or red crescent, chiefly, although there are other alternatives).
Now, let me take you to the battlefields of VietNam, circa 1960s, early 1970s. Information, imparted to combat medic trainees at the US Army Medical Training Center, Fort Sam Houston, TX and to other medical personnel at the Medical Field Service School on the same base, was that they were entitled to wear the white armband with red cross when serving in a combat zone and/or to paint a white circle, with a red cross within it, on the sides and rear of their combat helmets. They were told, however, in the next breath (and I know this because I heard it first-hand and I taught it myself at the Training Center) that it would very likely be used as a target. And it was! I proudly own one of those armbands, displayed beneath my Combat Medical Badge, but never worn.
Common wisdom was (and had been for a long time prior to VietNam) that the most effective technique to take in battle - short of mowing down the enemy - was to absolutely demoralize it. How? Kill or at least wound and disable (1) its commissioned and non-commissioned officers, (2) its radio operators (3) its medics (4) its guidon (flag) bearer. Bereft of leadership, communications, medical care, and a rallying point, it takes a strong military unit to survive and carry on. In identifying medical personnel, the GC's lofty goal of assuring their well-being made rifle fodder of them.
Mercy in war is, sadly, more often the acts of individuals - such as those heroically performed by Warrant Officer Hugh Thompson, Specialist Glenn Andreotta, both of blessed memory, and Specialist Lawrence Colburn, who intervened to bring an end to the My Lai massacre. It is rarely the act of an army.