Many astrologers unfamiliar with the ancient methods, approaches and terminologies are staunchly against the use of such terms. On the surface level it can appears as a very cut and dry good/bad distinction. If this is the case then ancient astrology is very dualistic and there is nothing subtle about it. This is not the case.
Without getting into the etymology of such terms let us explore how they are applied in an astrological context:
Firstly we have the notion of natural benefics and natural malefics. The former are Jupiter/Venus (greater and lesser benefics) while the latter are Saturn/Mars (greater and lesser malefics). The Sun is considered a benefic unless a planet is opposite to him or closely conjoined with him (the former results in retrograde motion while the latter occurs in something called combustion wherein the planet close to the Sun is said to be “burnt up”). The Moon is considered benefic while waxing and malefic while waning. Finally Mercury is benefic whilst configured with the benefics and malefic whilst configured with malefics (this is a simplification of the doctrine).
So does this mean that Jupiter/Venus are fundamentally Good and Saturn/Mars fundamentally Bad? Jupiter naturally signifies growth, abundance, harmony, faith, wisdom and has a tendency to lend a powerful affirmation to things in the chart. Venus naturally signifies harmony, beauty, love, friendship and agreement and denotes a bringing together, a bonding or connecting, a harmonization of that which she influences in the chart. From these significations we can thus understand that the term benefic, as it is applied in traditional astrological literature, denotes any planet or point (such as an arabic part, house, or aspect) which brings something forth into the world or confirms, maintains, and stabilizes it. In contradistinction a malefic (astrologically considered) does the exact opposite; it destroys, denies and prevents things from happening or remaining. Thus Saturn signifies limitations, labours, obstacles, difficulties and has a tendency to exclude/reject or prevent that which it influences. Mars denotes aggression, change, contentions and dynamic activities and thus has a tendency to sever or disturb that which it influences. Thus a quick example should suffice to see how this works out in practice:
When the benefics strongly influence the areas pertaining to children it means that it is likely or at least possible that the native will bear children because the benefics support and enable manifestation. If the malefics are the one’s more predominantly influencing then the possibility of children is greatly decreased or attended by much difficulty. Saturn produces excessive cold and dryness, while Mars produces excessive heat and dryness, both things are inimical to life. Jupiter produces, instead, heat and moisture, the exact conditions necessary for the production of life, while Venus is considered moderately cold/moist and by some moderately hot/moist, and is thus also supportive of life.
Simply put we might state that benefic implies growth and manifestation (Day) while malefic implies dissolution and change (Night).
Things become more subtle, more graded, more complex and ultimately much less black and white when we understand the concept of accidental malefics and accidental benefics. This is where dignity comes into play, which is something that modern astrology has largely ignored. There are multiple ways in which a planet is dignified/strengthened or debilitated/weakened. We won’t go into this now in any great detail but instead we will use to expand upon what was just said concerning malefics and benefics. Suppose Saturn, who is naturally malefic, is in his own domicile/exaltation (Capricorn, Aquarius or Libra), just to keep things simple. This increases his ability to produce things in accordance with his own nature and his fundamental nature is about limitation and constriction. Without limits we cannot exist, because limitation is the foundation of our individual existence. Without the bones, the fundamental structure of our body, we could not exist in an individualized physical form. Thus when Saturn can produce limitation, unhindered, he yields a strong foundation, a sense of depth, of form and focus and allows us to accept those things which we cannot change and to change the things that we, in fact, can. Thus when Saturn is well placed by sign/house he is functionally a benefic, meaning that he can bring into existence the affairs of those houses he manages by his rulership in the chart. His fundamental nature is unchanged and thus he will still produce delays and difficulties, but such experiences often result in depth and profundity. Thus if he produces a child, it may be a very challenging experience, but nevertheless a child can be born and such an experience of giving birth or raising the child will yield profundity and depth to the life of the native, or the child itself may have a powerfully Saturnian character, one that yields depth and a strong work ethic etc.
If Jupiter is debilitated, in the sign of Capricorn for example, then he cannot produce the wisdom and abundance he is capable of. Instead he may produce a lack of wisdom, a lack of the pursuit of truth, a lack of fortune or optimism which inevitably creates problems in the life of the native. Thus he may contribute to an extreme sense of pessimism or gluttony and no longer can he said to really be benefic; he is instead accidentally malefic within the life of the native. By this we mean he produces disharmony and disrupts equilibrium and such things are likely to lead to suffering and the subversion of the native’s goals.
We can apply the above logic to any of the planets.
Finally we have the concept of benefics/malefics based upon the houses ruled or the houses the planets are placed within. For example the lord of the 12th house, even when well positioned and well dignified, will generally produce challenges of varied sorts. A strong 12th lord generally means that the person is good at bringing things to a meaningful end, yet an ending is still involved and endings usually entail a crisis. They are, however, a necessary part of existence but the 12th house is malefic in so far as it represents an area of life which is not conducive to the birth/growth/maintenance of a given area of life. If a planet is in the 12th and is debilitated by sign and aspect then we get some serious difficulties in that area of life. If the lord of the 2nd is in such a situation then we can expect that the native will have many financial difficulties or be very fearful for their basic survival. The planet/sign and the rest of the chart will give us more details as to how such circumstances arise and play out.
Thus the malefic/benefic distinction is made within traditional astrology not as a classic good/bad distinction but instead as one which allows us to understand which areas of the chart are conducive to temporal/physical growth and personal happiness and which areas or antithetical to such things. No life is without a certain amount of harmony and disharmony, and the malefic/benefic distinction allows us to understand how that plays out in a chart and thus in a life.
It is important to understand that, paradoxically, the most malefic planets in a chart can yield the most spiritual growth, while perhaps the most benefic planets could be antithetical to that same growth. Yet again nothing is so simple and the whole chart needs to be carefully examined. Generally a weak planet needs to be supported or augmented by a strong planet in order for this to happen and various other factors also need to affirm the likelihood of this. Such is a discussion for another time.