In antiquity arson was among the most serious crimes in existence. Modern building codes and firefighting techniques have rendered fire in cities a relatively minor threat, usually only threatening the building that initially caught fire, but in antiquity a single fire could threaten most of the city. At Rome insulae were packed together, often sharing ply-thin walls with the buildings next door and made of inflammable materials (or, very frequently, just rubble with a thin facing of stone or brick). They were packed into neighborhoods as densely as possible, and often buildings competed with each other in height, meaning that besides the threat of a neighboring fire lighting yours up because cinders fell from their overhanging roof, buildings from across the street might set yours on fire the same way, especially since most of the streets in the city were narrow alleyways hardly wide enough for two-way foot traffic at all. Fire was such a serious problem in the ancient city that it was actually legal to knock down a neighboring house in order to save your own if you could prove that the fire was going to spread to your own from that building. Later emperors, beginning with Nero, tried to establish some rudimentary building codes, such as requiring buildings to have their own walls instead of sharing them with neighboring structures, or requiring the inhabitants of every dwelling to have firefighting equipment (so, basically, buckets) in the courtyard. At some cities, like Ostia, these appear to have slowly been effective as bit-by-bit builders adopted them--at Rome they were generally ignored. As such a serious threat arson incurred the highest penalty possible: under the Twelve Tables the punishment for willful arson was death by burning. The lex Cornelia de sicariis et veneficis contained a provision for arsonists who had dolus malus, "evil intent," a Latin legal term that applies to crimes that are committed willfully and with the intention of harming, rather than accidentally or without the intention of hurting anyone or damaging anything--accidental arson was not generally considered a criminal offense, but since the fire-starter still had culpa, "guilt, responsibility," for the result it was liable for suit under the lex Aquilia, which provided for lawsuits for damaged property. Later the crime was included in the provisions of the lex Julia de vi privata, which prohibited public and private violence and the carrying and stockpiling of arms. The lex Julia established the punishment of exile for most offenses it mentioned, but I'm fairly certain that the punishment for arson was still death, because there was an exception to the exile penalty in the law which I believe is supposed to include arsonists: under the lex Julia attacking someone's property (or carrying off a woman, I dunno why those are lumped together) was punishable by death.