We make light of hell with comedy, yet such planes exist. (Family Guy: The Fat Man's Going to Hell)
The universe in at least one sense is not "fair." When an intentional action (karma) is performed, it leaves traces in the mind called impulsions (javanas). These set the stage for the fruition of those mental, verbal, or physical deeds. They have the power to give rise to mental resultants (vipaka), fruits (phala), and even rebirths.
The Ten Courses of Unwholesome Action (AN X.206) are called "courses" because they lead like a corridor to rebirth in unfortunate destinations, the Downfall, the Great Waste, perdition, the hells, subhuman worlds of animals, hungry ghosts, serpents (nagas), demons (yakkhas), and hellions (narakas). Eight great hells are detailed in Buddhist cosmology.
It is often misleadingly suggested that hell is singular and reserved for the worst of the worst -- the dictators, mass murderers, and politicians. So people dismiss the idea of there being a hell or that they could ever end up there (it wouldn't be fair). But, in fact, there are many levels of hells appropriate to one's action(s).
However, just as with good karma, the results are disproportionate to the act. Harsh speech, killing an animal, holding wrong views -- IF they should come up at the death-proximate moment -- have the power to lead to rebirth in a hell. Even killing a single person could lead to rebirth in the most infernal netherworld -- depending on the victim. For example, killing a blameless, harmless person (i.e., an arhat or enlightened individual) leads to just such as rebirth, as does matricide, patricide, instigating a schism in the Sangha, or even holding the wrong view at the time of death that karma is without efficacy.
The great unfairness, if it may be called that, is how easy it is to end up there. On top of that, once reborn there, the lifespan to exhaust that karma feels like an "eternity." It is not. There is no literal eternal damnation. But this is not as comforting a thought as it may seem because a day in any of the hells is far too long. How long then thousands of years or an aeon? Time flys when you're having fun in human and superhuman worlds, but it crawls on broken glass in the hells.
