Cult Leaders are by definition aligned with their own faction.
For more information, please refer to the Cult page.
Some moderators will force the Cult Leader to unavoidably die if they attempt to recruit scum. This would make them Weak.
Some moderators will allow another Cultist (usually the one that has been in the Cult longest) to replace the Cult Leader if the original one dies. However, this makes it easier for the Cult to continue growing, which is not usually desired.
Some moderators will force all allied Cultists to die upon the death of the Cult Leader in order to prevent the remaining Cult from essentially being a Mafia without a kill.
Some moderators will allow any recruited town power roles to retain their original powers despite their alignment change. When this is allowed, the leader is often called an Improved Cult Leader