You always say there was a way out of it and I/we don't want to see it, but that's not true. We just understand why Marco thought it was necessary to kill Karen. See:
He hadn't spoke with her, he didn't knew Aftran and that she may would enter the peace-movement. All he knew was that there was a little infested girl, and that aftran had been in Cassie's had. He knew, that Aftran knew everything about them and that the yirks would infest or kill them if they ever get to know who the andalite bandits are. So what options did the have? They had two:
1. Starving the yirk out
2. Kill Karen
If they would have starved the yirk out there would be the one big problem: What to do with Karen?
Hiding them with the Hork Bajirs or the chees?
The Hork Bajirs could not care for a child, they could not! What should Karen eat up there? Not to mention that that little girl would live with aliens that looked like monsters. So this wasn't really a possibility.
And the chees? I think I said often enough that they just could not hold her back if she wanted to run away. So if they chose this option there was always a risk.
The second option was killing her. It would have been a really cruel decision, but it would have been reasonable.They just could not risk anything, to much depended on them. They could not give up anything for just one human.
And lets think it through: If Karen would have escaped, the yirks would have found her, the animorphs would have been destroyed and the war was over, which means the every person on earth would be made a controller. Also Karen. So, would she had gained anything?
The fact that you really can't understand where I'm coming from frightens me. This is why I hate humans so much.
Even if you won't believe me: I actually CAN understand what you mean. It's not like I think "oh okay, kill her, that's okay". No, I don't think so, for me life is sacred, but I know that the reality is that sometimes you need to make sacrifices, if you want to or not.
Can't you see where I'm coming from? Can't you see how I feel about this? Really, there was a way out. You just don't want to even consider it could be a good solution. You want to protect the human race, fine, but at what cost? Any cost? If we lose what makes us human in the first place, is there any point in fighting anymore?
If there is a good solution then tell me which. But don't come with the Hork Bajirs or the Chees. I think I made it clear that I can't see a good solution in it.
And to
You want to protect the human race, fine, but at what cost? Any cost? Well, you want to save one human but at what coast?...