Fret slots should tip away from the leading edge towards the tuners ideally, so the nut should sit upright. Is the bottom of the slot angled or the bottom of the nut? Or is it just a bit sloppy in the slot? hard to tell from the pics. A thin shim of some kind behind it might help.

I have a piece of 3mm aluminum that I have glued sandpaper to the edge of with superglue and trimmed to that thickness. It's really useful for tidying up nut slots and getting the bottom flat and perpendicular to the sides. Just go easy and check frequently or you'll end up having to shim under the nut.