What did he do that made you consider him especially heinous?
For me, it was forcing Kiyomi Takada to burn herself to death, without a shred of sympathy or regret. I remember reading it and being taken aback, and seeing Light in a new light (no pun intended).
This was where he truly became a Villain Protagonist for me, as absolutely everything he did was by necessity; yes, even killing L and Rem, as they were both threatening his life. But this was so cold-blooded that I was viscerally disgusted with him. To think he even cried out her name when he was dying.
Oh. And taunting Naomi Misora. I could see that threatening someone who is on the verge of (unwittingly) taking you down would be satisfying, but taunting her and beholding her horror was not.