I think, morality aside, from practical point of view, killing Sadeas isn't that certain to be a positive thing. He's more trouble dead, then alive.
I think WoR reveals Sadeas to be a shadow of the man who tried to save his King, when Assassin in White came. For all his words about Dalinar losing his mind and strength, I'd say it was more of Sadeas projecting what he himself been feeling. Sadeas is constantly lying to himself about his motives. He's shown himself to be a coward - sure he used to be a great swordsman, but now he just butchers foes, who can't harm him. His army is ill-disciplined and I'd say half of it would flee at the sight of voidbringers. He's good at manipulation and treachery, but I doubt they'd be very effective now.
On other hand Dalinar's position has grown very strong. Three High Princes now fully support him as High Prince of War, his words were proven right, he controls quarter of all shards in Alethkar, captured Uruthiru and he's got FOUR RADIANTS on his side. Sadeas can puff about ''taking it all'' from Dalinar, but just operating the Oathgate needs a Radiant. Of which only Shallan doesn't have personal reasons to hate him. You can't control Uruthiru without Radiants. His chances of killing Dalinar went to minuscule, now that Dalinar has stormlight, and High Princes who would chose to believe his lies (which are obvious, but some Alethi High Princes would just stick their head into the sand, no doubt) could only be cowards or idiots.
So Sadeas alive would be a nuisance, which Dalinar would be able to crush.
Sadeas dead on other hand would give credit to the lies against Dalinar. If people suspect that Dalinar had him killed, then maybe the ridiculous conspiracy theories he's been spouting could be true. Somebody will step into Sadea's place, he must have an heir (who could be a puppet for Sadea's wife, who'll actually rule). Dead Sadeas could be used to persuade the remaining High Princes, that they must weaken Dalinar to protect themselves and that his talk of honor is subterfuge and he's not above killing his rivals.