This is Nagito Komaeda's character song from the album progressive - 漸進 - [progressive -progressive-], written and performed by Megumi Ogata, Nagito Komaeda's CV.
English Lyrics by tumblr user marchen-v-friedhof
[http://marchen-v-friedhof.tumblr.com/post/109315022824]
Title: poison - 劇薬 -
poison -deadly drug-
Character Song of Nagito Komaeda
Artist: 狛枝凪斗（CV：緒方恵美）
Nagito Komaeda (CV: Megumi Ogata)
Album: progressive - 漸進 -
progressive -progressive-
Picture by Pixiv user いん
http://www.pixiv.net/member.php?id=252952
Tumblr: http://seki-in23.tumblr.com