I think Adis outright said that the Sin Purge Ritual would just kill them anyways, so leaving her alive doesn't have a peg to stand on. That's our (posthumous) expert on it. I've already given my reasons in the 'clubhouse' so to speak and they're pretty much the same as everyone else's. Death is the only release-let's just make it quick and merciful.

Plus, I think that interviewing her at night is a terrible idea. I don't think Mew's protections work that well, with the possibility of getting basically ganged up on by all six Stalkers.