Warui Slime Janai yo 悪いスライムじゃないよ

The phrase warui slime janai yo, or rather, warui suraimu janai yo 悪いスライムじゃないよ, meaning "[I'm] not a bad slime," comes from the RPG series Dragon Quest: it's a catch-phrase used used by NPC slimes to tell the player they aren't going to fight him.

Game: Dragon Quest IV, Doragon Kuesto Foo ドラゴンクエストＩＶ

• Tensei Shitara Slime Datta Ken
• Origin

Tensei Shitara Slime Datta Ken

I'm only posting about it because the phrase was recently used in that isekai slime anime.

Anime: Tensei shitara Slime Datta Ken, 転生したらスライムだった件 (Episode 6)

• hajimemashite
  はじめまして
  [Nice to meet you.]
• ore wa suraimu no Rimuru
  俺はスライムのリムル
  I'm Rimuru, the slime.
  • suraimu no Rimuru
    This is an apposition. (see No-Adjectives.)
• warui suraimu janai yo
  悪いスライムじゃないよ
  [I'm] not a bad slime, [you see!]

This was an obvious reference to the Dragon Quest series that may have escaped a lot of people that never played the game.

Origin

In case you didn't know about it, Dragon Quest was originally a classic Japanese RPG with random battles, and slimes were one of the first monsters you fought against. Although pretty weak, they were monsters, so they were your enemies.

Starting with Dragon Quest IV, released on Nintendo in 1990, slimes could be found around towns and villages as NPCs, rather than just as enemies in random battles. Such slimes could be talked with, rather than fought against, and the catch-phrase was probably used to indicate they weren't going to fight the player like the other, bad slimes.

They were good, peaceful slimes. Supposedly.

The first instance of the phrase was in the 4th chapter of Dragon Quest IV, in the Koomizu-mura コーミズ村, or "Kievs" village. It's shown below:

Game: Dragon Quest IV, Doragon Kuesto Foo ドラゴンクエストＩＶ

• *"ijimenaide kure yo~.
  boku wa warui suraimu janai yo.
  ＊「いじめないでくれよー。
  ボクは　わるいスライムじゃないよ。
  pls no bully,
  I'm not a bad slime.
  • ijimeru いじめる
    To pick on. To bully. To be cruel to.
• Menu:
• komando コマンド
  Command.
• hanasu, jumon
  話す, 呪文
  Talk, spell (cast heal, etc.).
• tsuyosa dougu
  強さ, 道具
  Strength (stats), tools (items).
• soubi, sakusen
  装備, 作戦
  Equipment, strategy (reorder party members).
• tobira, shiraberu
  扉, 調べる
  Door (opens a door adjacent to you), investigate (opens chests under you for some reason).
  • And yes, you actually got THREE different commands—talk, door, investigate—to do stuff where a normal RPG would have only one command that does any of the three depending on what is in front of you.
• The brief window on the bottom right is the name of the characters, their HP and MP.
• Minea ミネア
• Maanya マーニャ

In the English version of Dragon Quest IV, called Dragon Warrior IV, the following translation was shown instead:

• Don't hurt me! I'm not a bad Slime!

For reference, this is what a slime looked in battle:

Game: Dragon Quest IV, Doragon Kuesto Foo ドラゴンクエストＩＶ

• Aian アイアン
  (character name.)
• kougeki 攻撃
  Attack. (this doesn't work sometimes.)
• nigeru 逃げる
  Escape. (this never works.)
• bougyo 防御
  Defend. (who uses this lol.)
• dougu 道具
  Tool. (items, also known as yakusou 薬草, "healing herbs." Practically useless after you get a healer in your nakama.)

As you may suspect, using the adjective warui 悪い, "bad," before nouns that refer to people implies they're morally bad, just like in English. Normally, though, you'll see it in the form of warui hito 悪い人, "bad person," or warui yatsu 悪いヤツ, "bad guy." This warui suraimu phrase just follows the same format.

Manga: Yotsuba to! よつばと！ (Chapter 1)

• sono hito wa
  warui hito kamo
  shirenai zo
  その人は
  悪い人かも
  しれないぞ
  That person might be a bad person.

References

• 【ボク　わるいスライムじゃないよ。】 - wikiwiki.jp/dqdic3rd/, accessed 2019-01-21.
• Dragon Warrior IV Speeches - rulerofevil2k via gamefaqs.gamespot.com, accessed 2019-01-22.
  Source for official English translation. Note that it's full of spoilers, so only check it if you don't have absolutely any plans on ever playing Dragon Quest IV. Although, to be honest, I really don't recommend playing it, because it's a mind-boggling grind fest that will leave you wondering how did an entire generation of gamers survive the NES era without getting their brains fried. Seriously, you have to start from level 1 over and over? And sometimes you can't walk 10 steps without getting into 5 random battles? It's like there are zubats everywhere. EVERYWHERE. Just go toward a direction and keep pressing the same choice button incessantly, wishing all these stupid monsters die already so you can get out of the cave, or tower, or whatever. It really drives you insane. 0/10 I actually got a headache. It took 20 hours to get to chapter 4. I'm not even joking: the whole weekend. If that slime was on chapter 5 I would've actually given up before I got to see it. Go play Chrono Trigger or Terranigma or something on SNES instead. Don't touch NES RPGs. They're evil.