User blog comment:Kajune/Why did Lilith want to kill Sam?/@comment-26982641-20160403064508/@comment-24924499-20160403123427

Of course she wanted to free Lucifer. That was her and Azazel's motive all along. That's why she hired Ruby to be her double-agent. That's why she provoked Sam into killing her by laughing at him and mocking him when he hesitated about killing her. Her "I'll stop breaking the seals if you Winchesters sacrifice yourselves" speech upon trying to strike a deal with Sam was all a trick.

Technically, she didn't need Sam to kill her for any particular reason other than the fact that back in season 3, the ability to kill a demon was rare: only Sam, Ruby's knife and angels were known to be able to do so. Why she didn't have Ruby to just stab her with her knife when it was all done is a mystery. But, considering this is a TV show, that would've been too lame of an ending. Lilith needed to go with a bang, so why not have Sam kill her.