Yeah, that free moving center post on the adapter IS an issue for me too.
First let's start by saying that I'm completely sold: I love the killer, the vapor, its temp and the flavor is all I want for vaping and I am about to order my 2nd one as soon as it's available again.
That said, here is the story, at least for me:
If you open the juice control, it create a gap between the center post of the adapter and the base of the killer (normal, you unscrew to open). If on the mod you lose contact.
At this stage you can tighten the killer on the mod effectively pushing the connector post of the mod into the 510 adapter which pushes the center post in, closing the gap between the killer and the adapter's post and establishing contact again.
So far so good you'll tell me. NOT. by screwing in the killer, the side air hole of the adapter gets underneath the mod's connector thread. The result is a tighter draw and most annoyingly for me, the air passage being covered, it makes a light whistling noise when I draw.
I contacted Leo about it and he admitted that with some mods the connector is deep indeed and this might happen. He advised to mod the adapter by filing lightly above the adapter's threads on top of the air intake to flatten it. I very carefully did that and it worked.
That said, I'd hate to effectively ruin a piece of my atty at least aesthetically (even if not seen but this is how I am with my stuff

)
A spring loaded center post on the adapter would be awesome if achievable as not all mods (unless perhaps the very high-end ones) can make adjustments on the center post of their connectors.
I think that to become potentially the best atomizer of its class in the world, this part requires a revision.
Either this could be achieved with some sort of spring-loaded center post on the adapter or in the base of the killer itself or have the top part of the killer just slide onto the its bottom section (no thread). Both cylinders would have their holes aligned and to control the liquid flow you would only twist the two to open or close completely the holes.
Makes sense or am I the only one struggling with this issue?