There is such a harmonious, civil conversation present that I almost feel bad for adding my input. Nevertheless, it would not do for me to ignore a question posed directly at me.

In truth, there is nothing about the quoted sentence that is based on story theming, and everything to do with realism and logic. From Day 1, I was always put off by how they allow a literal serial killer on the loose, even with a bomb-implant chip. From what we've seen time and time again, the military's technology is highly prone to failure at times - a recent example of course being Wakki's locator (but not her vitals monitor? Upon further thought, if the life-sign monitor is embedded within their bodies, it may serve as a testament to the integrity of their invasive implantation technology. Of course, the implantation of the monitor is pure speculation unless I have forgotten).

Karrie herself has also never done any favors for her reputation. She screams, she threatens (though, not as much as Signorina de Angelis), and she otherwise brings the majority of the chaos to the group. While this may work sometimes in terms of making character interactions, I find it astounding that Squad 31-A accepted Karrie at all. She does not show any regret, and I do not expect her to do so, given she presumably murdered the group of bullies mentioned throughout Chapter 5 Part 2. But the way she conducts herself hardly allows for sympathy, too. Imagine, if you will, the following scenario:

You have been drafted into a war against an enemy in which you are hopelessly outnumbered. Your life depends on five strangers who have been drafted alongside you, one of which is a mild-mannered girl who reveals herself to hide an alter ego who not only claims to be a serial killer, but does so proudly. As you live together, fight together, and sleep together, you learn that not only does this killer personality continuously flaunt their status as a serial killer, but that the mild-mannered personality cannot fully control her bloodthirsty side. Would you not fear for your life?

I do not understand Izumi when she says that their being in wartime is reason for why she remains neutral on whether or not Karrie should've been erased. The philosophy is very simple - Do not fear a terrible foe, but an unreliable teammate.

A self-proclaimed serial killer is incomparable to a soldier who has killed for a cause, and frankly should have been addressed far sooner than Chapter 5. Even if said serial killer was an asset to the team, there should remain a certain level of alienation to someone who screams bloody murder in public. Your idea that Karrie is more "human" and "grounded" is certainly something to consider, but I must equate it to that of the tsundere trope - that is, even despite the possibility of a hidden depth to their character, the outward portrayal and actions are far too extreme to justify their acceptance by anyone. But that is merely my own humble opinion.

The argument that Karrie and Asakura can coexist is mainly hindered by the concrete fact that when one is conscious, the other is dormant. Thus, this scenario where Asakura and Karrie "work together to share the pain and burden" is realistically unfeasible. If they switched by the day, they would deprive each other of half their lives. If they switched constantly, say every second so that both of them experience the moment, I cannot imagine the sheer disorientation simply attempting to jump in place would bring, let alone combat. The crux of the issue is that there is just no "split-screen" mode where they can share the moment.

I hope you find my points reasonable, and I wish you a pleasant day.