The Radiant orders seem to have different views on what is right or wrong. The Windrunners or Skybreakers may not accept Adolin for what he did, but there are other orders would be perfectly fine (even happy) with his actions (Brandon confirmed this). Many of the Surgebinders/Radiants we've seen so far have had questionable killings. I believe Shallan took the quicker/easier path by killing her father. He was already incapacitated and they could have just restrained him and got the proper authorities as her father did just kill his wife and nearly killed her brother. The Highprince already was highly suspicious of her father and would have probably removed him if someone came forward with his crimes. He deserved his fate, but there were other options Shallan and her family probably could have taken.
Jasnah has used assassins to murder people and has purposely place herself in danger with the intent of killing criminals. Ym (interlude) was an accomplice in a murder. We don't know much of the details, but he did claim to be ignorant of the poison he gave to someone though. Dalinar has indicted that he has killed people that didn't deserved it (during his Blackthorn days). He has apparently killed people that just had too much too drink and probably insulted him or something.
- I really don't want a love triangle scenario to happen, though the story does feel like it will have Kaladin/Shallan pairing at some point. Kaladin getting together with Shallan while she is still with Adolin or causing them to break off the betrothal doesn't seem to be very honorable and makes them both look bad (especially if Kal and Adolin become true friends). Then Adolin getting jealous or having extreme rage towards Kal and Shallan would be awful as well. I don't particularly enjoy reading/watching this type of stuff. I do, however, really enjoy Kaladin/Shallan (I like Adolin/Shallan too), but if they get together, I hope that Shallan and Adolin would've already broken up for their own reasons. I really like how Kaladin and Adolin relationship progressed in WoR and hope they can continue it. - As for Adolin going bad, I think that killing Sadeas will probably causes a chain of events that will bring Adolin to a dark place, similar to Kaladin and Shallan having to go through terrible periods in their lives (probably nowhere near as bad). I think that there is a good chance that he will emerge though it, just like they have, and become a Radiant himself. I really don't see him going completely darkside, as that would mean going against the people he loves most (Dalinar, Renarin, Navani) and it doesn't seem to fit what we've seen from the character so far. Even if Kaladin and Shallan happen soon, would that really make Adolin go darkside? For a woman he's hasn't been seeing that long (about a month only so far). Maybe if they were together for a much longer time (or married) and he was really in love with her, I could understand. At this point, it doesn't seem to fit for a guy that is used to going through so many relationships, even though he really likes Shallan and wants it to work out. The whole rage/Odium theory is interesting and very well could be the thing that causes him down the dark path. However, I think we haven't seen enough occurrences from Adolin to say it is Odium influencing him yet (unlike Shallan's father). Anger and rage towards Sadeas was completely understandable and I would be more worried if Adolin was becoming enraged all the time over any little thing. Even Kaladin was enraged at Szeth before their second fight. It will be interesting to see the consequences of Adolin's actions and where it will take his character. ...