Eh, any master tactician would know that keeping the old ruler around is a rebel insurrection waiting to happen. Keeping him alive gives them hope, and there's always a chance that someone who's still likely very well connected could escape from your dungeon. If you deopose him and let him live, then you've got a citizen walking around demanding special treatment and since this is Noxus, likely rallying your forces against you.

Then again, since Noxus's hat is unchecked ambition and respecting only power, I'm assuming that being the ruler of Noxus means that you kick so much ass that you need a special shoe polish designed for butt-stench. So he might actually respect those who have the gall to someday challenge his authority... he just keeps a good eye on them and constantly sends them off on suicide missions.

I'm under the opinion that the two goals are VERY closely related. I feel as though Swain, when Jarvan IV was captured; an event that involved SEVERAL Noxian champions, purposely allowed 'The Exemplar of Demacia' to be taken back. This would result in Urgot's "death" at the hands of Garen, and likely in a public outcry over General Darkwill's inability to secure a victory for Noxus in spite of the great and powerful military leaders he had at his disposal.

Swain's self-induction into the League puts him in direct conflict with the Crownguard children, Garen and Lux; as well as Xin Zhao, Demacia's Seneschal. With the threat of Swain looming overhead, Jarvan IV may become cocky and enter the League himself to reduce the Noxian threat. This gives Swain EXACTLY the opportunity to be the leader and genius that Darkwill could not: He'll murder Jarvan IV on the Fields of Justice in cold blood; he'll do it in the name of Noxus, and this will turn the people and, more than likely, the military force of Noxus on its leadership.

TLDR? Swain murders Jarvan IV and succeeds where Darkwill had failed and Darkwill's leadership crumbles beneath Swain's charm and unbridled cunning.

The only problem with this lies with the fact that people don't die permanently in the Fields of Justice.

Have you forgotten? Swain has connections. The spell that stops champions from dying suffers might suffer some "temporary interference".

As of Leblanc's judgment we are now aware that Swain is no longer "Swain". He traded his "identity" for a position of power near the General and, perhaps the most important take-away from the judgment: Swain was not only close with Leblanc, but a member of the Black Rose himself.

What this means is that it might not be terribly difficult for Swain to interfere with that magic so that he could murder Jarvan IV. However, he's got to be crafty because Lux, Garen, and Xin Zhao; as I mentioned earlier, are all there as well. If the case isn't that he wants to murder Jarvan IV on the Fields of Justice, then he may be trying to create a situation that would call for Boram Darkwill to enter the League so that he could try to take them out in one fell swoop.

Although I don't think Swain would make a plan so ill-fated; one that puts him in with few allies and a boat load of enemies. Rather, he'll attract Jarvan IV to the League, murder him, then use his newfound popularity to oust Darkwill.

Oh, and he'll take all of the female champions as his consorts because he's a charming individual.

Do you guys know that Bird movie from Steven Spielberg?
Guess I know how Swain will manage to take over Demacia and Noxus.

There is something that bothers me about Swain existence or more likely getting me suspicious of. We don't know anything about him as a child. But there's one thing that I hope Riot can reply (although it may sound silly).

We all know that Jarvan and Swain are eternal rival to each other, not through by nation but personally. But I noticed that both of them almost shares the same hair colour, and most of the noble blood shares something like that.

even though both of them don't know it or probably Swain knows it:

Are Jericho Swain and Jarvan Lightshield IV actually brothers?

Hold the phone! M.Night Shyamalan's directing the Lore Department?

Riot would never do anything lore-wise that would remove a Champion from the League though. That would cause some issues with the gameplay. Unless we start using Jarvan clones.

The King of Demacia is Jarvan the third. Swain wants to kill Jarvan the fourth, which is likely a personal vendetta. Swain doesn't want to eliminate the ruler of Noxus, per se, (Boram Darkwill) but at the least, he wants him out of the way so that Noxus can come under Swain's rule.