The closest thing to Thelemic Compassion that I have seen is in the writings of the Chogyam Trungpa. In his book, Cutting Through Spiritual Materialism, he describes something called, Ruthless Compassion. He is very careful to differentiate it from conventional notions of compassion, what he refers to as "grandmother's love". But he also states that it is a dangerous practice for reasons that may shed light on why it is the "vice of kings".
@Chogyam Trungpa said
"To the conventional way of thinking, compassion simply means being kind and warm. This sort of compassion is described in the scriptures as "grandmother's love." You would expect the practitioner of this type of compassion to be extremely kind and gentle; he would not harm a flea. If you need another mask, another blanket to warm yourself, he will provide it. But true compassion is ruthless, from ego's point of view, because it does not consider ego's drive to maintain itself. It is "crazy wisdom". It is totally wise, but it is crazy as well, because it does not relate to ego's literal and simple minded attempts to secure it's own comfort.
The logical voice of ego advises us to be kind to other people, to be good boys and girls and lead innocent little lives. We work at our regular jobs and rent a cosy room or apartment for ourselves; we would like to continue in this way, but suddenly something happens which tears us out of our secure little nest. Either we become extremely depressed or something outrageously painful occurs. We begin to wonder why heaven has been so unkind. "Why should God punish me? I have been a good person, I have never hurt a soul." But there is something more to life than that.
What are we trying to secure? Why are we so concerned to protect ourselves? The sudden energy of ruthless compassion severs us from comforts and securities. If we were never to experience this kind of shock, we would not be able to grow. We have to be jarred out of our regular, repetitive and comfortable life styles. The point of meditation is not merely to be an honest or good person in the conventional sense, trying only to maintain our security. We must begin to become compassionate and wise in the fundamental sense, open and relating to the world as it is.
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Q: This ruthless compassion sounds cruel.
A: The conventional approach to love is like that of a father who is extremely naive and would like to help his children satisfy all their desires. He might give them everything: money, drink, weapons, food, anything to make them happy. However, there might be another kind of father who would not merely make his children happy, but who would work for their fundamental health.
Q: Why would a truly compassionate person have any concern with giving anything?
A: It is not exactly giving but opening, relating to other people. It is a matter of acknowledging the existence of other people as they are, rather than relating to people in terms of a fixed and preconceived idea of comfort or discomfort.
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Q: Isn't there a considerable danger of self-deception involved with the idea of ruthless compassion? A person might think he is being ruthlessly compassionate, when in fact he is only releasing aggressions?
A: Definitely, yes. It is because it is such a dangerous idea that I have waited until now to present it...
... At the stage of which I am speaking, if a student is to actually practice ruthless compassion, he must have already gone through a tremendous amount of work: meditation, study, cutting through, discovering self-deception and sense of humour, and so on. After a person has experienced this process, made this long and difficult journey, then the next discovery is that of compassion and prajna. Until a person has studied and meditated a great deal; it would be extremely dangerous for him to try to practice ruthless compassion."