What is a manga about a mc where he joins a dangerous cult? In the cult the cult members are told to do bad things like murder or anything illegal. I prefer if it had a religious cult or ideas. Or when thec is a spy in the cult

The only one I know of is Sensen Spike Hills. His mother is a "true" believer type member of the cult, so he is dragged in with her. No one seems to be translating the rest of it through.

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How lucky I am to have something that makes saying goodbye so hard." -Winnie The Pooh

"As through this world you travel, you'll meet some funny men; Some will rob you with a six-gun, and some with a fountain pen." - Woody Guthrie

No man chooses evil because it is evil; he only mistakes it for happiness. - Mary Shelly

Everything happens for a reason and that reason is usually Physics. - Anonymous

Well Shoujo Gensou Necrophilia maybe?

Not what you are looking for, but very good: Sugar Dark - Umerareta Yami to Shoujo

sidooh fits for the first chunk of it i think