But the British, so illogically merciful to men who were wounded, were utterly merciless to anyone who was not

From Waterloo A Near Run Thing by David Howarth. Page 122.

Every time the French came thundering up the slope, the man next to Morris said, ‘Tom, Tom here comes the calvary’ - and every time he got the word wrong. Morris believed the French were turning the British guns round and firing at his square at point-blank range, and he even had a mental picture afterwards of French artillerymen with the slow matches in their hands. Nobody else reported that, and probably he was mistaken. But shells and grape-shot certainly came from somewhere, blasting lanes right through the square. The men on both sides of him fell, the man who said ‘calvary’ got a ball through his thigh and died of it later, and Morris had a piece of cast iron which lodged in his cheek so that the blood ran down inside his clothes. The old captain, thirty-two years in the army and never in action before, was horribly frightened - so Morris said — and came to him several times for a drop of something to keep his spirits up: but he was blown to bits before the day was out. Yet every time the grape-shot blasted gaps in the square, the infantry closed them, dragging the wounded into the square and throwing the dead outside, before the cavalry could ride through them.

French lancers were killing what British and German wounded they happened to see. A mounted swordsman could not easily reach a wounded man who was lying on the ground, but a lancer could do it with horrifying ease, a mere gesture of the heavy lance as he rode by. It angered the British who saw it almost beyond control - such a blatant disregard for what they thought were the decencies of war. It angered them just as much when their allies did it. Gronow saw a colonel of the French Hussars fall under his horse, and while he struggled to free himself two Brunswick soldiers ran out of the neighbouring square and took his purse, his watch and his pistols - and then they put the pistols to his head and blew his brains out. A shout of ‘Shame!’ went up from the British square. But the British, so illogically merciful to men who were wounded, were utterly merciless to anyone who was not, whatever his situation. Sergeant Wheeler, still in the sunken lane beyond the Nivelles road, saw nearly a hundred horsemen coming along it at full gallop: they must have passed thorough the squares, and rather than ride back through the musketry fire were looking for a way round the end of the British line. But the land had been blocked by branches. Unable to climb the banks, and unable to turn because of the others pressing on behind, they came to a halt jammed together as tightly as so many horses could be. Wheeler and his companions, manning the road block, fired a single volley. By the time they had loaded again and the smoke had cleared, one man, and only one, was running away. One other was taken prisoner. For curiosity, Wheeler went to see what had happened to the rest. The men who were shot dead were lucky, he thought, for the wounded horses, plunging and kicking, were finishing what the musketry had begun. He could not see a single man he thought was likely to recover, and as he had other business to attend to, he left them to their fate.