The key set is levelled horizontally at the front edge so that each key travels the same amount of distance down and up. In a properly regulated grand action when the key lever is at full stroke(down as if played) the key should show level with a spirit level as should the hammer stem of the corresponding key.

Should piano keys be angled?

It almost looks like it's a keyboard sitting in an acoustic piano shell. There should be no tilt. You want 90 degree angles.

Are piano keys slanted?

All piano keys slant. They are flat (parallel to the floor) when they are fully depressed. What throws some folks is that the cheek blocks (wood blocks at either end of the keyboard) are also slanted on some pianos making the keys look flat.