First, thank you very much for playing our game, it means really much to us :D
You probably lost because there were other enemies nearby when you killed one, as that leads to you getting noticed and losing. A good indicator for that is the red circle around you, as long there are no other enemies in that circle you shouldn´t get noticed.
The key gets a use in the second level, before that it has no use. The level you are talking about probably is the tutorial level, where we put every available item so that you later know which objects are pickup-able.
The USB "corrupts" the pc it´s used on. If an enemy works on that pc he has to visit the boss to get it fixed. The rest it does is for you to find out ;)
We´ll absolutely rework the tutorial after the jam so that the game is much easier to understand , especially about mechanics like getting caught (red circle)
Again, thank you for the feedback :D