Even though it's true, Karthus may not be a bad guy. Don't know if his intentions are evil for sure.

Also: *dies to protect inhibitor*

nocturne?

Swain- is master of noxus. they wanna rule everything. But everyone hates them. and would probly band together to end noxas perhaps? so they arent getting anywhere. applies to most, if not all Noxus people.
Malzahar- Wants to bring the void, which is evil. but he is probly just a pawn in some grander scheme of things.
Mordekaiser- Maybe. depends on what, if anything he leads.
Veigar- seeks to end all conflict on Valoran by bringing all of the city-states to their knees, regardless of their affiliation. So he is kinda a good guy in a wierd way. Still awesome though.
Varus- no. just no.
others
Cho 'Gath- Is a monster. wants to destroy everything. Im pretty sure its kept in the same prison as Nocturne. And he probly gain nonsense amounts of powers if let loose from eating any and everything. So he is REALLY a evil thing. but might be some random pawn for something else.
Brand- Wants to burn everything. And he would given half a chance. so he is Evil.
Nocturne- Is EVIL! it wants to kill everything. pretty sure its not for gain of any kind. IS REALLY EVIL!
Renekton- wants to kill things. And isnt he almost a god or something?
Karthus- Isnt really evil, but isnt good. hes Neutral. maybe.
Hecarim- Leads immortal ghost army? i would say, yes is evil. Started Destroying Villages for no known reason. So yes, he is pretty Evil.

My vote on the Most Evil Villain in LoL would have to be Nocturne. Practically immortal, and is a true Nightmare. not to mention i pretty sure it kills for the sake of killing.

Cho gath,

be weary of the day cho gath becomes tired of the league.

No Morgana?

Just pitching in so I dunno if someone covered this:

Swain already got what he wanted: total control of Noxus. In fact, watching that conspiracy take shape was pretty much the whole focus of the Lore updates to begin with.

Most of the other villians suggested thus far have only a single personal motivation to draw them to the League, usually a rival that they want to kill.

What would throw me would be if some senior summoner of the League itself turned out to be the main villian. I mean it has all the hallmarks of a Supervillian's plan:

-- collect all the world's greatest warriors in one place
-- opposing nations with conflicting political and moral philosophies brought to heel by a handful of extremely powerful individuals.
-- scare all the other nations (at least nearby) into ceding near total military control
-- a missing and enigmatic original leader who vanished under mysterious circumstances
-- a plethora of monsters and ancient arcane forces "suddenly" coming out of the woodwork.
(seriously: Nocurne, Brand, Xerath, Zyra, Anivia, Fiddlesticks, Trundle, Skarner,etc. are all nonhuman entities fused with some elemental force into an entirely new being via magic to say nothing of the various flavors of undead and robots in circulation)
-- Even in the midst of THAT there are forces openly representing some otherworldly force that's threatening to one day tear the world asunder (The Void, The "Glorious Evolution")

I wouldn't be surprised if the missing former head of the League or some other senior Summoner was enatcing a plan reminiscent of Ozymandias from Watchmen to unite Valoran against some threat to all Runeterra at any cost. The obvious enemy would be the Void but that seems a little to easy really.

There's a neat essay in the "Why we Summon" thread that sums up why the League simply CAN'T last so from a story perspective it has to be being put together for a PURPOSE and given that Valoran's greatest warriors can now butcher eachother and confront malevolet elemental creatures without dying, I'm betting that purpose is one of TRAINING.

There's another neat thread trying to suss out how magic works in Valoran that (while containing a heavy dose of parody) did establish (for me at least) that magic in Valoran isn't neatly divided into categories and levels but is a bigger, wilder concept than that.

Maybe all that magical pollution is leading to more and more beings like Nocturne, Xerath and Zyra awakening and the League is training warriors to confront what should happen if Runeterra's magic gets even MORE out of control. (Which is basically the plot of Generator Rex but as far as plots go it wasn't a bad one.)

Morg's "evil" is pretty small-scale, simply wanting to best Kayle in the League and go home to be praised as the superior sister.

I say Malzahar, for mostly the reasons people have already stated: Even if he is not the true leader of the Void, we don't have the true one, and he is still the general overseeing it until his masters appear. Cho and Nocturne may as well be his monster pets he let loose in the meantime.

Swain and LeBlanc are iconic evil, in that they are enemies of Ionia and Demacia, but they too would fight against the Void. Hell, with champions like Riven around who realize there is such a thing as "too much evil", despite being loyal to Noxus, would even team up with the "good" nations to fend off the rest.

Shadow Isles champs (Morde, Hecarim, and I guess Karthus?) seem even more neutral in the fact that they enjoy power over their opponents rather than power in general; no opponents, no power.

Poor Renekton, getting his ass kicked by his bro both in-game and lore-wise

Renek can best his bother, just if he is jungle, and Nasus gives farmed: Renekton needs to say on the pool to prevent death.

Ok, on topic, her is food for thought. Ryze scrolls is Karthus's song, and that if anyone sings the full song, the world is destoried.

No, no, and thrice: no.

There is a deeper game at play.

Swain arranging for the death of General Darkwell and taking control of Noxus was the first step in a much larger plan. LeBlanc is currently imitating Jarvan IV and the whole conflict which brought about The Crystal Scar was manufactured by LeBlanc, Swain, and the rest of The Black Rose.

The manipulations and scheming of The Black Rose have already brought about the complete destruction of a town, and brought the first open conflict since the formation of the League of Legends itself!

Note: Swain is the visible mastermind with a clear(er) purpose, so he is the 'villain' over LeBlanc.

Note2: Malzahar's/Hecarim's/Karthus'/Rammus' plots have not yet been developed anywhere near as completely as Swain's/LeBlanc's.

I believe that Viktor will be a great villain. He dosen't want political power, he doesn't want mindless destruction/killing, no, Viktor only wants to complete what he believes as the "glorious evolution", turning the weak flesh into superior constructs. Though the whole jayce debacle, I still believe Viktor will be a great villian. Cybermen for the win!