The bull named 'Guapetón' - 'Handsome' - gored a 43-year-resident to death in June during the fiesta of San Juan in Coria Extremadura.
But revenge was quick and the bull was corralled into the town square where a resident raised his shotgun took aim, and floored the beast with a shot to the head.
The death of the killer bull on June 24th was filmed and the clip uploaded on YouTube.
WARNING: Video contains graphic footage
The spectators, many of whom can be seen filming the incident, cheered and clapped when the bull dropped to the ground.
But the shooting has been declared illegal after an investigation by Spain's Guardia Civil following a complaint by animal rights campaigners.
The shooting was carried out in front of residents and local police and with the full knowledge of town hall authorities and without "due caution for public safety" according to the report compiled by the Guardia Civil.
It is therefore being investigated as "a serious infraction" of the new Citizens' Security Law which forbids the firing of weapons for entertainment purposes. The town hall is now facing a fine of up to €30,000.
The report states that the bull "after an hour and a half of encierro (running in the street) was in a state of anxiety and exhaustion and was killed by a festival goer armed with a shotgun".
There are strict regulations governing the killing of a bull in such circumstances outside of the bullring which insist on a controlled death in the presence of a trained veterinarian.
The Partido Animalista (Pacma), a political party campaigning for animal rights, which made the complaint, welcomed the Civil Guard report.
"It was an illegal death carried out in front of dozens of residents, the local police and with the knowledge of town hall authorities," said a statement from Pacma sent to The Local
"(The report by the Civil Guard) sets an important precedent that could help bring an end to this kind of festival involving the death of an animal for entertainment purposes," the group said.
"Pacma is committed to bringing about fiestas that do not involve animal abuse and substitute the death of animals with ethical alternatives," it added.