Why is scourge evil?

If you read the manga or Bluestar's Prophecy (Super Edition) it will tell you.

But Tigerpaw (Tigerstar as an apprentice) attacked Scourge when he tresspassed ThunderClan territory. Tigerpaw almost killed him but Bluefur (Bluestar as a Warrior) stopped him. Scourge was just a kit then. Tigerpaw said: "You'll never forget me!" and he didn't. Scourge wanted revenge, so he killed Tigerstar. He pretty much turned evil because of what Tigerstar did.

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Adding on to that, Tiny (Scourge) was always picked on by Socks and Ruby, his older siblings. His ambition, you might call it, is what got him killed by his half-brother, Firestar.