I have not faced anything akin to your example, but I had a couple of experiences similar in that I had to choose to 'pull the plug' for my youngest brother and for my father.
How did I feel afterwards?
Not guilty. Just honoured that I had been able to make the decision which would save them from huge suffering.
As for your example: originally we are not told who the 5+1 are, so we cannot decide based on how we feel about them.
Would I choose to kill the one? Yes, I would. Would I feel guilty?
Why should I? The decision was forced on me and I had to decide to sacrifice someone....so, I might decide it makes sense for me to find the bastard who put me in this position and kill him/her (a thought...was that person you?)
I'm sorry you had to go through that. And I'm glad you saved them from more suffering.
It's a compelling reason why you shouldn't feel bad. But there are people who are more sensitive than others and may not be able to bear the burden of their decision. But the important thing is to survive it.
To know that it was the best decision you could have made at that moment and with that knowledge. And to know that you could have done it differently is not important. And of course, learning from experiences.
don't kill the messenger! 😁