I've always thought that are trying to infiltrate the sith/dark side to get rid of it in secrecy. I think obi wan wanted Luke to kill Darth Vader because he thinks he killed his father not knowing that is his father. thus avenging his father's death by killing Darth Vader and taking down the empire. But I don't think obi wan thought Vader was gonna tell Luke he's his father which changed everything obi wan was trying to do. Idk that's just my little take on it.