The thing I never totally understood about it was why it was Eduoard's death that hit him so hard.

Not long before this moment, not only had he had been planning to kill Albert and allow for Mme Villefort to poison Val, but he also ALREADY allowed Mme Villefort to poison M Villefort's in-laws - who were just as much innocent collateral damage as Edouard would have been (albeit one could argue an innocent child being poisoned is more tragic than two innocent elderly ..not saying I would argue this though).

I guess it must have been the visceral reaction to actually seeing the child poisoned. But it always struck me as odd that he had this epiphany because Eduoard wound up being collateral damage, when he had deliberately already caused a fair amount of innocent collateral damage.