Why was Bertuccio upset when he went to the house at Auteuil in The Count of Monte Cristo?
Bertuccio had once attempted to murder someone at the house in Auteuil.
Bertuccio's brother, who had been a soldier in Napoleon’s army, was murdered by royalist assassins in the city of Nîmes. Bertuccio visited the public prosecutor of Nîmes (Villefort) to seek justice, but Villefort, a royalist, was unsympathetic to Bertuccio’s story and turned him away. Bertuccio swore revenge on Villefort.
Villefort transferred to Versailles, but Bertuccio followed him there. By spying on him, Bertuccio discovered that Villefort often came to the summerhouse in Auteuil, where he kept his mistress. One night Bertuccio waited for Villefort in the garden behind the house and witnessed him burying a box. Bertuccio stabbed Villefort, leaving him for dead, and grabbed the box, thinking that it contained a treasure. Instead it contained a baby, which had been smothered but was able to be resuscitated. Bertuccio eventually took the baby home with him and raised it with the help of his widowed sister-in-law.
He was upset because that was the place where he witnessed a murder. Villefort murdered a baby and buried it underground in a box. Well he didn't murder the baby but he put it in a box and buried it under the ground. He was also upset because after he witnessed that there he "murdered" Villefort by stabbing him (so he thought).