There are some heavy planets in astrology that can have a detrimental or even destructive effect on the Sun and these are mostly the outer planets - Saturn, Uranus, Neptune, and Pluto.

Saturn is pehaps the worst, with its rigid or severe character which can be a real downer on the happier, more generous Sun. This has been observed both in synastry and through transits where the heavy-handed Saturnian fatherly approach comes down on what it sees as a childish or undisciplined Sun. Just study the Capricorn/Leo relationship and you'll get the idea. Capricorn admires Leo's strength and warmth but not its bombastic, egotistical, or childish nature, and can be very critical and demeaning of Leo here, especially when Leo does not produce the results it boasts or promises. The conjunction or same sign aspect is especially critical, and to a lesser extent, the opposition, trine, and even sextile. I've seen plenty of relationships where "good" Sun/Saturn links are readily jumped into only to become traps or burdens. It's really bad when this happens in permanent relationships like family or marriage.

Surprising how the good aspects in this case are worse than the the bad ones. This is because the good aspects bring together two inimical planets whereas the bad aspects (square, semi-sextile, or quincunx) keep their distance. This works for all enemy planets. In an ideal chart where every planet is in its (true) home sign, planets ruling fire or air are kept distant from planets ruling earth or water and that's the way it should be.

Uranus can be a mixed blessing. In bad aspect (square, semi-sextile, or inconjunct) it can be seriously irritating or disruptive, but in good aspect, Uranus can make an ideal partner or ally. Both are masculine or positive planets, and can hold their own, so the positive should outweigh the negative, even under adverse aspects. Adverse fixed signs tend to be the most uncompromising.

Neptune is not intentionally bad but can seriously drain or weaken the Sun, especially when in conjunction, opposition, trine, or sextile, and it doesn't have to be by degree. I've seen fire Sun signs acting very Piscean in nature when Neptune is simply in sextile to their Sun by sign. This can be in the natal chart, through synastry, or by transit. Neptune in the same sign as the Sun is of course the most powerful aspect, but simply being in the same gender is sufficient enough. Neptune is currently in Aries, for example. Observe how Aries or the other fire Suns are becoming more wishy washy or laissez-faire. I can even feel this as an air sign! Best when Neptune is in the opposite gender to the Sun, either nataly, by transit, or relationship.

Pluto can definitely pervert, degrade, or demoralize a Sun in its own gender, unless the Sun individual lives by very high moral standards. It's all or nothing with these individuals, it's either the high road, but more often the low road. Trump, for example, has Pluto sextile his Sun, trine his Moon, and conjunct his Mars and Ascendant (by sign), making him a very Plutonic character. He plays very serious and deadly games affecting great masses of people, sometimes for good and sometimes for bad. Pluto is amoral and does what needs to be done even if it means breaking the law.

The Taurus side of Venus can weigh heavily on the Sun but the Libra side mitigaytes this. In ancient times, Venus was a rogue comet-like terror in the skies as described by Sitchin in his Twelfth Planet book, making it somewhat malefic. Even the Moon (or is it Rahu) can dampen the Sun via its eclipses.

There may be two more planets that can negatively affect the Sun which have not yet been discovered or confirmed. One is Vulcan/Isis/Rahu/Black Sun, supposedly (but not necessarily) between Mercury and the Sun, which has a descisively negative effect on the Sun. Just study the Virgo/Leo relationship: Virgo being a sign ahead, sees Leo's shortcomings or weaknesses and points them out, which Leo does not like, and so the relationship sours. In the same way, Vulcan, wherever it may be, is a threat to the Sun. The other planet is beyond Pluto or even Eris and often referred to by many names such as Planet X, Planet Nine, Nibiru, Tyche, Nemesis, etc. and is also considered negative or a threat to the Sun.

All of these bodies, known and unknown, are "Sun killers" because of their negative or disruptive effect on our precious Sun, both physical and astrological. Their dangers are minimized or maximized by both their orbital distances and their astrological aspects to the Sun.

In short or summary, all feminine sign ruling planets -Moon, Venus, Vulcan, Saturn, Neptune, Pluto, can be considered inimical or bad for the Sun whereas all masculine sign ruling planets Mars, Jupiter, Mercury, Uranus, and Earth/Ascendant or whatever is the true ruler of Libra (Eris?) can be considered friendly or supportive to the Sun.