Alright the problem here is that anakin and obi-wan both need a sith lord to do their battles with.
Obi-wan needs one to avenge his master's death however this doesn't necessarily need to take place in the first film.
and anakin needs one to.....no....actually i don't think he does need one.
If maul took on sideous type of role in the films and obi-wan had to do battle with him in the first film, lost, then lost again to him with anakin's help in teh second film, then confronted him in the third, it would have been fine.
anakin wouldn't have had to kill anyone to be honest, his move to the dark side would have been based on a similar event that occured with mace windu, but with them having more of a duel
but then this brings into question the balance of power questions.
alright, when mace comes in with kit fisto and some of the others i think anakin should already be there, he takes out fisto and some of the other knights and comes just in time to save the emperor from mace.
i don't think anaking necessarily needs to kill a sith lord to take his place since ultimately it wasn't the reason he killed him.
even based on this, obi could defeat maul but maul is still a live and when anakin is about to go all dark, the emperor tells him to go and 'retrieve' maul but when anakin goes to get him, he kill him instead and asks to take his place etc...