Funny you should ask, I just went through this a couple weeks ago.
The trick is to remove the cover before installing the killflash. When I tried to install the KF with the cover in place the KF cross threaded every time.
I removed the cover and cleaned up the threads on the KF. After that the KF threaded in with zero effort and seated deep enough that it touches the cover when closed but doesn't interfere.
On a side note, the KillFlash degrades the image and dot quality significantly. I wouldn't try to use it in low light or indoors. If it's dark enough that the light from an M68 might give you away, it's too dark to use it with the KillFlash.
I posted this a little while ago
www.ar15.com/forums/topic.html?b=1&f=74&t=146887 and got mixed answers. I've since retired mine to the hall of great ideas and other failed experiments.
YMMV.