I've done a lot of things. Killed a lot of people. Senseless murders, mercy killings, I've done it all and it never did, and still to this day does not, bother me. I'd likely do it all again if I could go back. But if it came down to it and the Fates decided somehow my brothers or sister had to go, I couldn't.
Even the cruelest of people have their limits.
Whoever does this. Whoever offers themselves for this trade. They have to know that, even if while they are acting as Death they are able to remain impartial, to do what needs to be done...if this is, in fact, a temporary deal and you come back to yourself...you'll carry that with you. You personally did not kill them, not exactly, but I'd imagine it very much feels all the same. It taps into things...things that may make even the most innocent and pure less so in the end. You won't be the same. And you won't ever escape it.