不埒(ふらち) is a vivid word meaning "immoral," "shameless," or "brazenly reckless." It has a stronger, more old-fashioned and literary feel than simply saying "bad."
In the lyric 堕ちていく不埒な夜 ("a shameless night that keeps falling"), it describes a wild, improper night spiraling out of control. Its dramatic nuance and unusual kanji make it a memorable word to learn from this song.
Janne Da Arc's "GUNS" is a late-night swagger song, and it does not pretend to be anything else. The narrator wakes up on a "灼熱の夜" [scorching night] and admits he is "たまにタチの悪い俺" [sometimes a nasty piece of work]. タチが悪い is a handy idiom: it literally means "bad nature" and is used for people or things that are trouble.
He goes out with "自慢の GUN 忍ばせて" [my prized GUN hidden on me], eyes up a woman who looks easy, and sets out to make the night "アダルト" [adult]. The gun is never really a gun. Lines like "今すぐ引き金引かせて" [let me pull the trigger right now] and "GUN もハヤク乱れ打ち" [the GUN fires wildly, fast] make the double meaning pretty blunt. It is cheesy, and it knows it.
There is a drug thread too: "ハイになってトリップしたら" [once we get high and trip], then "お次は灰になろう" [next, let's turn to ash], a pun on ハイ (high) and 灰 (ash), which sound the same. Two lovers are "絡まり合う毒蛇の様に" [like poisonous snakes coiled together].
The chorus repeats "まだ帰さない" [I won't let you go home yet] and then flips to "まだ帰れない" [I can't go home yet], plus "いつもはこんなんじゃない" [I'm not usually like this]. That last line is the classic excuse, and it is a little funny next to everything before it. For learners, the pair 帰す (to send someone home) and 帰る (to go home) is the most useful thing in the song.