Beliar is very straightforward about his ambitions. As the Gothic universe's patron of death, destruction, and mutation, he makes it abundantly clear that his only goal is the total annihilation of all things. If that isn't enough to dissuade you, let's take a closer look at what happens to those misguided souls foolish enough to worship him.
Yes, his adherents are initially granted power and wealth, but that's just the beginning of their Faustian pact. You see, over time, Beliar's faithful tend to undergo what could charitably be called "an evolution"--a patch of scales here, a sprinkling of radiant demon-fire there. And before long, the full blown transformation into a hideous half-demon, half-human abomination is complete. While on some level this may seem kind of awesome, I imagine it's a pretty tough sell to parents, friends, and lovers.