Gangsta (ギャングスタ) ED - Yoru no Kuni (夜の国) - piano version 

Let's be friends :3 / joshagarradomusic
Sheet music: (will unlock at 20,000 views)
I totally bounced off the first episode of this, couldn't get into the story and the visual style didn't really appeal to me.
But I got bored one day and gave it another try... woah, this anime really literally is gangsta. Dark as hell, too