Arson may not have been the intention if the liquid was lubricating oil.
As others point out, lubricant is hard to ignite, perhaps the intention was simply vandalism/messing up the place.
Or as suggested above it might possibly have been an accident.

Diesel fuel is hard to ignite unless soaked into a porous surface such as carpet, when it ignites easliy.
(a firefighter once told me that proffesional arsonists use diesel or kerosene, never gasoline. Diesel or kerosene burn fiercly and will often destroy the building, and murder any occupants. Gasoline is liable to explode and kill the arsonist)

I would agree that the street door should be locked.