Why are some pickups slanted?
Single coil pickups are angled in order to enhance the treble response of higher strings, and the bass response of the lower strings. Because single coils are naturally quite bright, they need this angling so that the bass response isn't too weak, causing the tone to be too harsh.
Are pickups supposed to be tilted?
Pickup tilt is normally adjusted to get the bass-treble output balance where you want it. Individual pole pieces are then adjusted to get the desired string-to-string balance. Overall height of pickups is adjusted to find the sweet spot, and also to get the desired balance between the neck and bridge pickup.
Why did Evh angle his pickup?
The pickup was installed at a slight angle to compensate for the differing string spacings of the Fender tremolo and the Gibson humbucker. Later he replaced the PAF with a Seymour Duncan humbucker.