So a bit vague but basically I typically don't bother with alignment outside of when it's used in mechs but session 0 I said my rule is no evil characters. No evil alignments I don't want to run a party with evil characters in it.

Anyway one of the players. An isolated assassin style character sort of discovered religion and the gods and of course made a beeline straight for Bhaal. Because of course.

I said this is an evil god. Just be aware that it isn't that its a perception of being evil. This is literally an evil god of ritual death.

So of course they were like yippee sounds great.

My issue is I want them to have the freedom to explore this and I hate restricting my players but at what point does praying and potentially worshipping an evil god become an evil act in and of itself?

I think there are some great RP opportunities here but I also don't want them to start murder hoboing their way through the campaign of I'm a bhaal worshipper what do you expect.