Tbh for me this is just bad writing because they don't connect the point. We probably aren't going to get and Isolde pov at this point, but all the people that say "Viego was a bad king and an abusive husband" are people outside of the relationship, the closest thing we got is Senna because she has "Isolde fighting spirit" or something like that.

But there are parts that just doesn't make sense, I may be wrong here but at the start is implied that Viego and Kalista had a good relationship then when she found the isles Hecarim started ploting and then Viego already losing his mind sent him to kill her.

So this is what I think, Viego was a bad king, but not a bad husband, otherwise why the heck would Isolde warn Viego and basically take the dagger in his place if he was just a bad person? Viego was young and powerful, which isn't a good combination but he truly loved Isolde, like madly in love, like the the first love of a teenager. And when she died he started losing his mind. We really need and Isolde pov but I also think this different sides of Viego is what makes him so interesting and if the event wasn't trash he would be the best character in the lore without a doubt imo