Dig the art style and the kickin' BGM. One-hit kills are pretty brutal: most classic platformers had invincibility frames for a reason. Here I assume the idea is to keep the player from invincibility-framing their way through the whole level without using the core mechanic, but I think that could still be done with a simple hit counter, say 3 strikes and you're out. This has the advantage of giving the player more time to explore the level before they're out completely and have to restart, which keeps the game challenging, but it a more fun kind of loop.
I also feel like, if you ever revisit this, it might benefit from being HALF stealth. Make the player stick to the shadows 3/4 of the time, but then give them a temporary power up or mechanic to let them carve a path of destruction through the rest as a reward. Build up of tension and then release. All good stories work this way.
Neat start though. Hope you keep working on it.