Hmm. How about this? Most people the suicide spirit kills are mangled in some horrifying way. But, if the spirit manages to get someone else to kill him/herself through the "suicidal urge," then they both vanish to another plan for the replacement idea?
Sounds workable, although I still prefer its replacement to leave a corpse that is resistant to raising it looks like we're heading for a vanishing corpse approach.
I'd like to give it a "Create Replacement" ability tied to its "Drive To Suicide" ability, though.
Shall we start roughing something out? Upon reflection, I prefer a modifier confusion for its suicidal urge attack.
Suicidal Urge: A suicide spirit can instill a temporary state of suicidal madness in a single living humanoid. Suicidal Urge has a range of (200?) feet and is a mind-affecting compulsion effect. The target must succeed at a DCX Will save or be driven insane for 1d4 rounds. Roll on the following table to determine what action the subject of suicidal urge will perform, the target will continue trying to perform the same action for the duration of the suicidal urge. If the subject dies while under the effect of Suicidal Urge they will become a suicide spirit (see Create Replacement). The save DC is Charisma-based.
D% Roll Behaviour
01-20 The subject attack the most dangerous opponent they can see with a melee weapon (or closes with the opponent if melee combat is not possible, ignoring any intervening hazards)
21-50 The subject babbles about death and self-harm, but takes no action.
51-70 The subject flees the suicide spirit at their top speed, ignoring any hazards in their path.
81-00 The subject tries to kill themselves with whatever weapon or implement is within easiest reach (treat as a coup-de-grace attack)
I'm also thinking Suicidal urge should have a limited use per day (1/day?) and do Wisdom drain or Wisdom damage as well as the self-destructive impulses, so it can cause permanent or long-term insanity.
I want it to be quite nasty!
Create Replacement: If a suicide spirit slays a humanoid via its suicidal urge attack, the suicide spirit and the victim's body will vanish, and the victim's soul is torn free of its corpse and cursed to become another suicide spirit.