This is surprisingly juicy for a beginner's game, impressive :)
The design, while simple, holds actual strategic weight, you have to walk around and stop walking, which creates an interesting conflict of interests, that's genius design. It still needs a little tweaking to not feel tedious, but the idea is very good.
Now, some criticism. I think bullet colors should be different for enemies and the player, in the heat of the action it may be hard to tell the difference for some players. The camera behavior is a little ufavorable in the tutorial, it focusing near the center makes the player not see what's near the edges until you're there. This of course stops being a problem once the proper game begins. I can say, this is one very spacy grave :D
As of bugs, once a dead shotgun enemy has shoot me, other than that it was clean for me.