Unity 3D Issues?

So, I have a Camera focused on a single Cube.
The Camera rotates around the Cube, based on the input from the keys.
All goes well, but when I start playing (both in editor and built version), the game renders wrong: instead of one single Cube around which I rotate, the "previous verions" of the Cube are still visible.
In the end, it looks like there's a single sphere with one face of the cube still potruding.
Any idea of fixing this? (It can't be my hardware - GTX 1070 + i5-7600K + 16GB RAM)

Edit: Even destroyed objects are still rendered. I got a temporary solution: render a black image on Stretch, which hides the previous frames, but it's very intensive on calculating power.