You first meet Sarevok at the conclusion of the "Open Hand Temple Murders" questline, in which you help the hollyphant Investigator Valeria solve a series of mysterious killings in and around Baldur's Gate. Killing Sarevok means sparing Valeria, Jaheira, and Minsc, along with all the other people Sarevok and his followers intended to murder. Neither choice affects the ending of Baldur's Gate 3, and becoming an Unholy Assassin only changes a couple of things in the short term, so it's better to defy and kill Sarevok.