I don't think it was ideological trends that made the push; rather, the material factors of war incentivized certain behaviors we now attribute to humaneness.
For example, allowing the capitulation and retirement of fortress garrisons. After the walls had been breached, there was nothing the garrison could do to prevent the imminent fall of the fortress and their deaths in battle. However, storming it would be quite costly to the army, since every soldier was someone they drilled and clothed and fed for years, and since these are largely absolute monarchs, you're flushing the personal wealth of the king down the drain if there's bloodshed after the walls have been breached.
Thus both armies have an incentive to minimize losses, and thus expense to the king; their solution was allowing the garrison to return to friendly lines, bearing its colors and arms once the fortress had fulfilled its purpose o delaying the army. There's nothing ideological about it, it's pure, even mercenary self interest. The greatest mercenary of all time, von Wallenstein, in the age of religious wars of vaunted brutality, preferred to settle campaigns through siege and negotiation, rather than battle, because he was the one footing the bill on the bloodshed.
The laws of war are dictated by two contending forces: military utility, and the enemy's capacity for retaliation. Wars are as brutal as is useful, up to the point where the enemy doing the same to you makes your methods suboptimal.