It was real estate speculators. Yes, he tried to build a huge palace on the empty areas of the fires aftermath, but he also lost a huge amount of money when his Domus Transitoria burned. When the people tried to blame Nero for building his huge palace on the area where these empty lots were, he had to find a scapegoat and he blamed the Christians. It wasn't his plans but it was convenient for him to blame them because they couldn't defend themselves. Rome was built mostly of wood and it was just a matter of time before there happened to be a huge fire. Nero went about rebuilding the city in stone but this is largely forgotten.
Suetonius and Dio Cassius pretty much say that he started the fire. Tacitus wrote that Nero was rumored to have sung about the destruction of Troy while Rome itself burned, but he states that this was unconfirmed by eyewitness accounts and that Nero was at his summer villa in Antium when the fire broke out.
we can't trust those reports...roman leaders hired their own historians to slander previous rulers and glorify the family that hired them, it was propaganda no different than what politicians use today...back in those times saying something slanderous about a relative of a ruling family was often fatal, so if it was openly said and recorded the person who said it felt safe in saying it, he had approval from the top... you know the expression "take it with a grain of salt", well when it involves historical roman reports about their leaders take those reports with a "shovel full of salt"...
I know...I am saying that Suetonius and Dio Cassius cannot really be trusted in this case and Tacitus is more believable (although he himself was a darling of the aristocracy in his time).
yeah it requires a lot of reading between the lines to sift out what was true and what was official bs...
Caligula your namesake was much maligned and probably deservedly so but there was no doubt added embellishment. to add to his notoriety...there was the reported war Caligula waged against the Neptune and the sea...he was said to have captured neptunes treasure which consisted of seashell...archaeological finds now suggest in his expedition to the north he built river ports along the Rhine to the North Sea as preparation for an invasion of Britain which his successor Claudius carried out...