Baal has a few behaviourisms that sets him apart from his two brothers, [[Mephisto]] and [[Diablo]]. He could have just killed Marius when he found him, taking the [[Soulstone]] but instead fooled him with the guise of [[Tyrael]] and listened to his tale. While this might have been a move to make sure Marius had not hidden the stone elsewhere (forever hiding its location if he was killed), it could be that Baal likes to play with his pray prey before destroying them.