Sune paladin turning to Malcanthet worship would be quite a twist; vain, lustful but no love at all. But then makes wonder why paladin was not allured by worship to Sharess (http://forgottenrealms.wikia.com/wiki/Sharess)? Only if said paladin was more kinkier and also having darker desires and emotions not fit neither Sune nor Sharess. However Oathbreaker paladin is literally paladin who has broken his oaths, so deity which he served he betrayed and turned his powers for selfish purposes.