I wouldn't say it's a bad concept, but it's not very good either.
The player is able to just stand still and the zombies will just die because his knife is facing them. This could be fixed by using a slash animation and only during a slash animation the zombies can be damaged. There also needs to be cooldown and knockback so spamming won't be an issue.
The player just gets deleted when a zombie contacts it. A death animation or even an automatic transition to reset the level would make it so much better.
No audio, very boring. The level lacks animation and feels very static, very dead. Adding details like trees shrubs and grass that sway can make this game feel more alive. There is also clear tiling of the grass texture, which looks unnatural. You need variation in your texture.
Even though the zombies have a death animation, they just disappear. Games like these need a lot of juice in them. Through in blood, corpses, and lots of screen shake (Not lots, but adequate). By leaving the zombie corpse on the ground with blood splattered can give the game a whole new feel and makes the player feel like they're in control of the world. If your concerned with object overloading, make levels or even just different areas.
A lot of feedback and a lot of work to do. However I do believe you are capable of making this much better.