Soul Slay isn't just an instakill. It is, in some ways, an overkill. The mind, or the soul, or some other vital thing is severed from the body; thus, the body is left mentally decapitated. They might still be alive after this, in fact, but a disconnect prevents the body from doing anything.

Soul-Slaying appends a matching Affliction to the unit, which prevents the unit from coming back to life. Should someone manage to circumvent the affliction, it disappears from the character sheet. Soul-Slaying prevents "resurrection" through the following:

• Changing Shape
• Immortality - the soul is punted to the Underworld, which it can't easily escape from
• Twiceborn - the soul is no longer anchored to your body, after all
• Straight-up Extra Lives, such as through Phoenix Pyre or a Crystal Heart

Soul Slay does not prevent the following from working:

• Being re-summoned - it doesn't actually slay the soul, as it turns out
  • The only exception is the Ritual of Rebirth; soul-slaying cannot bring someone back into their original body.
• Simulacrum - the soul isn't severed from your real body, and ends up in the Dreamwild instead of the Underworld
• Scar Souls - one of the souls takes the place of yours, or just takes the bullet

Thus, a Pretender God who is soul-slayed can still be recalled, and unique summons can still be reclaimed.

Virtually all sources of Soul-Slaying are resisted by Magic Resistance, by the way.