The novel is like an exception i luv it.
First he saved her. Then, he taught her how to live or move on and go back to the living land, he returned the light to her, and she was his light... So he wanted to be good, to fix himself for this light, for her!
He couldn't, so he preferred to leave the thing like it is before he screw it up more...
Its not like real suicide (for him) ... He was wandering; wandering the bottom of that lake... I think he was seeking a different world... Like the bottom is not the end, its just the beginning, a gate actually to a fresh start, new world... To be in a world where no one judges you or tells you what you must be!
He lift things or clues after him, becoz its not what you take, its what you leave. She got his message and what he lift... But he lift more in her soul.
I like how finch changed violet and made her better, and i respect his desire for not ruining it. But he could've do it by fixing himself rather than ending his life... After all love heals all the wounds not time.
Perhaps if he listened to violet, he would have been alive with her.