Veigar...Stop laughing...am EVIL

Singed has brutally killed untold hundreds of thousands, possibly numbering in the millions, directly or indirectly with his biohazards. He has no humanity, is completely insane, and cares about very very little other than himself.
His actions have caused the suffering of Soraka, Riven, Master Yi, and many others
Also according to the JoJ, he's a massive troll

Veigar. He is the Tiny Master of Evil after all.

I suppose it could be Cho'gath, Nocturne or Shaco.

Malzahar. He will bring the untold horrors of the void unto this world, and he will sacrifice your daughters to the void simply because he can. Don't look into his eyes, yo.

I don't think Malzahar is the most evil. He was driven insane because his second sight made him understand the void in a way that no other human ever had. The most evil champions are the ones that were not corrupted by anything but are evil just because. Shaco, Nocturne, etc.

Malz is just insane. Cho'gath is truly sentient and malevolent. Mordekaiser should be up at the top of the list too. He's basically the Lich King of LoL, if you pay attention the TT altars and other contexts.

Lulu duh

LeBlanc, she has been playing everyone for decades. Everyone is merely a doll that she is pulling the strings for.