Here's why:
* Regulus joined the Death Eaters: He was initially a young, misguided follower of Voldemort, believing in his promises of power and purity. He actively participated in Voldemort's campaign of terror, making him complicit in his evil deeds. This makes him, by many definitions, evil.
* Regulus changed his mind: He later realized the true nature of Voldemort and his horrific crimes. He actively sought to stop him, even at the cost of his own life. This shows a shift in his character and a desire to redeem himself.
* His actions had consequences: His actions, while heroic, still contributed to the deaths of many, including his house-elf Kreacher. He left a legacy of guilt and pain for those he loved, which could be considered morally ambiguous.
Ultimately, R.A.B. is a complex character: He was initially drawn to darkness but ultimately rejected it, sacrificing himself to protect others. It's up to each individual to decide whether they view him as evil, redeemed, or somewhere in between.
His character highlights the difficult question of redemption and the complexities of human nature.