I think there are some issues with the way "evil" clerics and deities are portrayed jn D&D.
There is a reason why none of the Greek city-states regarded Ares, the god of war, as their patron in the pre-Classical period: he was considered too crass, ugly, violent and fickle – unsuitable material for an exemplar. In contrast, deities such as Apollo, Hera, Artemis and Athena – representing various “nobler” human faculties – were popular in this role, despite their otherwise all-too-human foibles. Although not generally judged as evil from any conventional moral perspective, Ares did not receive the kind of devotion afforded other members of the Olympian pantheon. Sites were set aside for propitiation and sacrifice, but even the existence of temples dedicated to Ares is dubious – the ruins at the Agora in Athens seem to have been transplanted later by Augustus to honor Mars.
Where gods from our own mythologies are detailed as evil, and worshipped by wicked people, we are generally looking at the polemical opinions of religious and ethnic detractors. Where D&D classically diverges from our own history in this regard, is that evil people actually worship evil gods to effect nefarious, all-conquering or nihilistic schemes. This caricature is unfortunate, as it robs religion of the depth and complexity it deserves; the reason that we don’t have any real-world examples of people worshipping evil gods in any numbers is because for this to occur it would require some kind of mass psychosis. Perilous, angry, capricious, vindictive – yes; lots of gods have had these qualities in spades. Evil – not so much.
We could grope for an obvious significator of evil – say, infant sacrifice – but we know in reality that in the Bronze- and Iron Ages such practices were fairly widespread (which is not to say common), and that deities whom we should not ordinarily consider “evil” were often the target of such sacrifices. Catastrophic situations – and attempts to forestall some kind of natural or military disaster – may have prescribed infant sacrifice in many cases.
Evil devil-gods requiring a constant stream of sacrifices are a literary trope but absent from our actual history; there is a widespread pattern of emerging literate cultures projecting the practice of human sacrifice onto their neighbours in an effort to delegitimize them, whilst simultaneously repressing the memory of their own not-too-distant guilt. Even when we consider the extensive human sacrifices offered to deities such as Tlaloc, the Aztec rain and fertility deity, the judgement of “evil” is an external one; the actual relationship of the supplicant to such a deity is impossible for us to penetrate at this distance, but we do understand that Tlaloc was venerated in a number of seemingly contradictory roles. Whilst we might reasonably condemn human sacrifice as evil, it does not necessarily follow that Tlaloc (or Odin, or Yhwh, or some other target) is also.
Where deities – or monsters of deific power – are portrayed as unambiguously evil, their mythic role is one of adversary rather than patron. Mortals may be enthralled or deceived, but they are not willing or fully cognizant participants in the deity’s schemes. Insane cultists who worship evil deities are a time-honored device, but there are only so many times where this idea may be recycled before its repetition becomes a little farcical.
TL;DR - I don't know how to play an evil cleric plausibly.
