So that immoral, selfish, thieving and rotten Isabella survives nomatter what? Even if her selfishness caused countless death, murder, tragedy and despair for several years in Kirkwall? And if you give her to the Arishok she escapes justice again?
That concludes it - Isabella has the thickest plot armor in the dragon age universe. Id compare Isabella to Loghain but even Loghain could die, and atleast Loghain had good reasoning for what he did. Better than Isabella atleast 
tbh, I'd think that Leliana has thicker plot armor. You can 'kill' Leliana in the Gauntlet after defiling the Ashes, but she comes back in DA2 like, "lolno. You can't kill me!"
Plus, I wouldn't consider Isabela immoral. More like amoral, imo. Also, she doesn't kill people just because, and she did free those people who were going to be made into slaves. She values freedom of choice overall, but she doesn't really think about the consequences of her actions. The same can be said about maybe DA and ME characters, tbh.
Honestly, I view Isabela as being similar to Jack in ME2 and ME3. Her outlook on life has been shaped by her past in a... unique way, causing her to act a certain way in response. While Jack prefers to have her 'I hate everyone' attitude, Isabela has move of a 'anything goes' attitude. They're both similar in the way that they tend to distance themselves from others emotionally, having few 'true' friends (even though Isabela is more carefree) that really know them, if any (which would probably only be Shep in Jack's case). Their romances are similar in the way that they've both been burned in love before, so they're both hesitant to let themselves truly get close to anyone emotionally.
Anyway, I'm getting off track. I was trying to say that you should take Isabela's backstory into account when judging her character. People's actions are often influenced by their past, Isabela is no different.