Lilly's bad end?

[OtakuNinja]

I haven played it yet, so don't spoil anything!
What's the chapter called? In the 'library', it says I've completed all of Lilly's chapters.

[dwarduk]

There isn't one. It's just a lack of continuation, at least as far as I remember.

[Enemy |]

There's her Neutral Ending, but no "Bad" one.

[OtakuNinja]

There's her Neutral Ending, but no "Bad" one.

Alright, her neutral ending, then. How do I get it?

[Enemy |]

Don't talk about the letter you got by Iwanako in your date.
I think only that already sets you on the neutral ending, not sure though.

[dwarduk]

Lilly route flowchart (Linked for spoilers and because it's huge)

Follow the blue arrow to get to the Bad Ending. There are three choices that can affect it.

[demonix]

I haven played it yet, so don't spoil anything!
What's the chapter called? In the 'library', it says I've completed all of Lilly's chapters.

There isn't any chapter for the neutral end as it follows the good end and ends when Lilly leaves for Scotland which is why there's no extra chapters (it's also why some people can't unlock the last picture in the library since it only adds completion and nothing else).

As everyone else has mentioned, you just have to make different choices at certain points (the bit about Iwanako's letter in act 4 is one of those points)