I think a better way to handle this would be to remove killed comments from the normal thread view altogether, but make them available to anyone ("showdead" or not) by clicking on the comment's timestamp.
Oh I get it. That's actually similar to a change we've been meaning to make, but only for [flagged] comments as opposed to all [dead] ones. I'll think about the latter.
Personally I don't like this idea. If I've chosen to turn on "showdead", why hide dead comments from me? I've explicitly indicated I'd like to see them.
Because if we do it this way, we won't need "showdead" any more: "dead" posts will work naturally no matter what your settings are. There's no reason to have an advanced setting to allow you to click through to a dead comment; everyone should just be able to do it.
It also neatly dodges the problem of deemphasizing toxic posts on threads; the content simply isn't displayed in the thread, but is available in a more readable form just a click away.
It will also do nice things for the formatting of the threads themselves (flagged comments on nested threads won't take up much vertical space).