Ancient peoples believed that a mythological hound with three heads known as Kerberos guarded the Gate to Hell, causing death to all who entered, but scientists believe that volcanic CO2 was likely the culprit. This would account for the deaths of sacrificial animals that died there without ever having a human hand laid upon them.
Gates of Hell
In Rome, priests used to sacrifice animals in a theater of sorts that housed a cave or grotto leading down to what were thought to be the bowels of hell. Located in Greece, Italy and Turkey, these portals to the underworld had temples or Plutoniums built on them to serve as sanctuaries for the gods who reigned there (Pluto and Kore or Hades and Persephone, depending on your mythology).
As stated, these locations were sometimes the site of sacrifices, but not in the gruesome manner you're probably envisioning. You see, when animals were brought to the mouth of the "gate" they would simply lie down and die, leaving the frightened masses to believe the deaths were due to the breath of the hellhound Kerberos.
Kerberos's breath was actually thought to be the cause of the deadly vapors emitting from the sites. Talk about the ultimate case of doggy breath. Researchers now say most of these portals were situated on or very near natural vents in the ground where CO2 would rise. Sites that release lethal levels of CO2 are called mofettes.
What scientists found is that the concentration of gas was higher down below in the grottos and the theaters were raised, so that spectators to these events, who sat up higher and out of harm's way, would simply see the sacrifices led in and then mysteriously succumb, for no apparent reason, making the whole thing seem even more mysterious and supernatural.
The long and the short of it: Kerberos got a bum rap.
Source: Live Science