The way it's described seems very straightforward, yet I'm unsure whether it's actually the intended behavior. As we're left to guesses, I'll list what I think the MM is trying to say:
- Deadly Kiss is a quality of the character. It's described as "The kiss of a Succubus or incubus is an echo of the emptiness that is the fiend's longing for a corrupted soul," which makes me believe it's unable to suppress this effect, being tied to its own nature.
- Draining Kiss is an action, which has to be used in the RPG context, and unless they're doing so, they will simply kiss the other character normally (though with the effects of Deadly Kiss applying).
In the character narrative, a Succubus must choose to drain its victim's vitality, but they can kiss them without doing so.
I would also say that they might be unaware of the effects of Deadly Kiss, thinking to pleasure their victim, while giving their motives away in the process, or unintentionally killing the victim too early. They would, however, have to be aware of the effects of Draining Kiss in order for it to make any sense.
Indulging in a bit of story design, they would likely be furious about unintentionally killing someone they already had worked on for some time, becoming more relentless trying to get to their next victim, yet learn that they must use very specific methods and can't simply turn a human by physical pleasures alone.
They would possibly turn mad from not being able to fulfill their own desires eventually, becoming chaotic evil and using their abilities to cause pain and suffering, rather than strive for their original goal of obtaining someone's soul, and therefore becoming more of a mindless fiend than a calculating enemy.
The question remains how Deadly Kiss actually kills, if it's not by the same means as Draining Kiss. I would say (and this has no ground whatsoever, it's simply my guess, or how I would see it playing out well) that the pain caused by the kiss is inflicting damage proportional to the corruption caused by the Succubus (that the victim would be subconciously aware of).
This way, a Succubus interacts with low-level NPCs the same way it does with high-level heroes, in that it can't simply kill off any character, yet will be able to deliver a last fatal kiss to a successfully corrupted character, obtaining their soul no matter how powerful they are otherwise.
Also, the three betrayals are only listed as a tale's narration, while the rest of the paragraph about obtaining a victim's soul seems to be written more in a rules context (emphasis by me):
Beautiful Corrupters (...) A mortal bequeaths its soul to the fiend not by formal pledge or contract. Instead, when a succubus or incubus has corrupted a creature completely - some say by causing the victim to commit the three betrayals of thought, word, and deed - the victim's soul belongs
to the fiend. (...)
This means, as far as I would interpret it, that the Succubus has to corrupt a creature, which will usually involve that creature committing several actions that could in total likely be described as "thought, word, and deed", but could be as simple as a creature being fundamentally determined to taking revenge on a character by killing them. The creature could be seen as corrupted from that point on. The "word" component seems especially redundant to me, as a good character eventually turning on their friend and killing them should be corrupted enough, even if they never spoke of it before.