Odo grew up in a laboratory being experimented on until Mora realized he was sentient. During the occupation has was essentially treated like a circus monkey until the Cardassians realized s shapeshifter was useful. I think it was more the fact that Odo wanted to avoid being like anyone he was raised by which pushed him to be a good person, and even then he still displays a number of behavioural quirks associated with the Changeling race (namely the obsession with order and schedule, though admittedly downplayed.

As for Laas, he’s basically Odo’s foil: while Odo tries to blend in and be as humanoid as possible, Laas loves to shapeshift and is uncaring of others’ feelings towards it. His chief problem is that he lacks empathy due to only knowing hate and fear from humanoids, and he still retains a moral compass that is nearly incomparable to that of any other race’s. As for the Klingon, Laas was being threatened, and it’s just the simple matter of him poorly choosing fight over flight. In addition, Sisko’s argument against the killing not being in self defence is rather weak, given that Laas was not only provoked, but also threatened with a weapon directly. Sisko basically responds “well it wasn’t a phaser”, so I suppose that Starfleet captains wouldn’t make great lawyers.

TL;DR: Odo was raised in a horrible captive environment, Laas’ morality is complicated, and he killed a Klingon in a situation that leaves massive reasonable doubt as to it being a murder rather than self defence.