A really cool game it is! The gif really doesn't do it a lot of justice. For something made in such a short span of time this is rather impressive - it has a consistent aesthetic (well, arguably, the enemies I would not call fitting perfectly (not sure about the color choices, but it's just me) ), working gameplay and it adheres to the jam's rules.
What felt the nicest to me is the movement, it reminded me of older shooter games - think likes of Quake 2.
The starting gun felt kind of weak, mostly due to the fact that there's no hit indication on the enemies except for the blue laser particles (the gore is awesome, and, in my opinion, is very much fitting, unlike, perhaps, the enemies themselves) and the sound is just weak - again, a perception thing, but I can't offer any fixes because I often suffer from the very same kinda problem.
Oh, and also the music sounds good to me.