Who killed Lionel ***possible spoiler***

I think it's just as likely that Lionel killed himself. He did mention it as the only way out during his last meeting with Louis. And Louis gave himself away to Sibella in front of the fireplace, then tried to cover it up by making his "confession" sound like a joke, but of course Sibella didn't buy it, and Louis had to suspect she didn't. Louis and Sibella were involved in a sort of game of poker in which he at first bested her, knowing that Lionel had no clue about her infidelity, so I think she just continued the game by hiding his suicide note. Notice that at the trial, she never directly answers the question about whether there was such a note; she just says that the police never found one.

This film is very precise in its allusions - the only uncertainty being in the ending (did Louis retrieve his memoirs before they were discovered and read by someone else?) - but its subtlety is delightful.

As clive-ihd says, Sibella knew very well that Louis had killed a number of d'Ascoignes and in the prison scene she made sure that Louis knew she knew.

But kdmagnusson is correct. Lionel undoubtebly committed suicide exactly as he told Louis. Quite possibly he was already dead before Sibella had returned from visiting Louis to tell him of their 'discovery.' That Sibella concealed the suicide note is shown by the evasiveness of her answer in the trial. Her initial reason for concealing the note might just have been revenge for Louis discarding her in favour of Edith. Possibly it was only later that she realised she might also use it to her advantage as a means of becoming duchess herself.

I haven't watched in a little while, but doesn't Sibella say something like "producing a suicide note written in Lionel's own hand"? If she did say this (I don't recall at the moment), I assume that means she found a suicide note, but hid it so she could "play her hand" with Louis.