The main thing I think could be improved is the level design. I see that you have the common instinct to make branching paths in levels, but you actually should avoid that even though choosing a path seems interesting on paper. In practice you’re just turning your level into two levels and people will think they have to backtrack to get and see everything. Branches should be for secret areas or secret exits. Also I think one-hit-kill games should be more forgiving with checkpoints.