Many curious people are out there, who have come across runes and perhaps even heard of their magickal uses in ancient times.
Some are into astrology, or casual horoscope readers. Some are new-age or general spiritual types with an environmental bent. Others are deep into self-help, positive energy, hypnosis, NLP, and so forth. Some are Christians with an interest in old European ways. Others have other religions or no religion. And others yet are "alternative paths", pagans, wiccans, Asatru, Rodnovery, Druids, or other heathen revivalists.
Which of them should learn rune magick?
Leaving out any personal agendas, the answer should be clear - ANYONE can learn it, and theoretically anyone should be allowed to learn it, so long as they harm none. It's a very powerful and stand-alone system, without any need to learn other types of magick to practice it. It can affect vast changes in your life.
That said, there are those who would use any system to harm others, so inevitably it follows that there are some people who should never learn rune magick.
1. Those with evil or malign intentions towards other individuals or humanity as a whole. People addicted to their own rage, desiring to take it out on anyone who crosses their path. Humanity will not benefit in any way if those learning the runes are the same ones who only seek to use them to curse and negatively influence their environment. It's true you can take that path, but the result is not pretty for anyone. Many times did the Norns weave a sad fate for an aggressor without crying upon the person - think not that the fruits of your negative karma and tainted wyrd will be any more apologetically delivered to your doorstep.
2. Those who wish to use magick to exploit or harm others for personal gain. There is no shortage of scammers and con artists out there. Aside from creating "runic" orders and societies to entrap and exploit people, they may simply be looking to rune magick as a means to step on others and steal what is not rightfully theirs. While using runes for personal success is not necessarily bad, it must be remembered that the balanced runenmeister must be looking out for the benefit of more than just himself. Pure greed will tarnish the name of runemagick just as it has tarnished most religions and spiritualities. And the karmic debt will only breed revenge.
3. Racist or hate-oriented groups and individuals. This should be an obvious reason why someone should not learn runes. Leaving aside the debates on the merits of Volkisch thinking and national revival/solidarity among commonly related peoples, the real undesirable tendencies are those of the blind bigots who end up co-opting runic symbols to promote the crudest form of racist politics and mob violence, combined ironically with unfathomable willful ignorance, even of one's own culture and history, and the most degenerate of music (we use the term loosely here). Frankly all runers who truly love their folk (however you define that) should not want to be associated with criminal blind hate of others or with common brainless street thugs, whatever the race or color. Such is not the way of Hávamál, nor is it the way of the rune sagas. And none of the Lore ever posits that one race of humans is above another in any way - rather it distinguishes the heroic and honest from the fools, cowards and oath-breakers.
4. Extreme feminists (oddly, there are quite a few of these in modern magickal circles). This attitude is especially common in certain "pagan" groups comprised mostly of angry middle-aged divorced women who by some means have come to believe that every one of their sufferings was due to a man in their life, or simply all men in general, rather than their own choices. The desire there is not for wisdom but for "revenge" on men by any means, physical or psychic, perversion of the lore included. While women per se have always had a very important and welcome role in Norse magick and ritual, this sort of insane anti-male hate was completely unknown to the ancient Norse - such bigotry completely flies in the face of Nodh, or karmic need for every man and woman to accept responsibility for their own actions and reality, and also implies blatant disrespect for the sacred Nordic concepts of Orlog and Wyrd (natural law and cause-effect flow). Prone to abuse runes to construct unprovoked malevolent curses against men or "captive women" (i.e. any normal woman who doesn't ascribe to their bigotry), this vile lot should never be taught the runelore under any circumstances once identified.
5. Coercive "cults" and groupthink societies. Beware of groups that call themselves "brotherhoods" and "holy orders" and claim to be the only ones to possess the secrets of the runes. Beware groups with contracts and excessive fees, or which claim that you need to pay a great deal of money before learning any of the "mysteries". Learning the mysteries primarily depends on the person's mind and soul state, not on the size of their wallet. And while fees and donations can go a long way toward helping make instructors more feasible and better-trained, at the end of the day it takes time and study on your part, not merely cash. How much result you get depends on how much time and practice you put in. In fact those who cough up the most at once, often end up learning the least, due to expecting a "quick fix" which always disappoints.
6. Expropriators, profiteers, and bowdlerizers of lore (i.e. Ralph Blum, pop-Theosophists, pseudo-Qabalists, many Wiccan authors, Sweat-lodgers, Bahais, pop-psychologists, pop-astrologers, various watered-down western "Reiki" and "Tao" schools, etc). Obviously people exploiting and stealing from various esoteric traditions worldwide to sell a shallow, empty book or DVD are not the sort of folks you want to go to for rune knowledge. Likewise, rune masters should not take such people as students in the first place! Everything which is fed to them disappears down a black hole, and what comes out instead is meaningless trash designed to sucker the next... well, sucker.
7. The deranged or suicidal. Inevitably, as with the racists and hate-criminals, every time one of these "useful idiots" of organized Churchdom commits a shocking violent act, it's not the actual person, or their childhood tormentors, but rather their private interests that get blamed, whether it's a video game, a certain novel, a particular music band, "goth-culture", the Occult, or even Runes. We sadly live in a scapegoat society of dilettantes posing as experts, warm fuzzy conformity, flight from accountability, and self-medication, where people always blame neutral things for inspiring an act of violence against innocents, rather than the poor judgment of the person abusing them - as if people are no longer in control of their actions. Well, in the special case of the mentally insane, this may actually be true. Which further should drive home the point that runes should not be taught to people mentally incapable of acting responsibly. Such people are incapable of fulfilling even the most basic of oaths and will sooner or later drag rune-magick (and even Germanic spirituality itself) through the mud if given the chance.
These are, of course, not by any means the only people who should not learn rune magick, this is far from an exhaustive list. However keep in mind that if you truly want to get something good out of it, you have to put something good in. And if you are willing to accept that, and take pleasure in affecting positive change in those around you, even in the most subtle and seemingly insignificant ways, beyond their notice, then you have the makings of a true runer.
Keep in mind, this does not mean being some sort of wishy-washy wiccan/hippie fluffbunny, or that you can never construct negative bindrunes or curses against those who have betrayed or harmed you - sometimes you may need to - but rather, that this should never be your primary purpose for being a student of the runes. Runers are not expected to always "turn the other cheek" or "harm ye none", as we accept the reality that conflict is an inevitable part of life - but at the same time there still is a baseline ethic a true runenmeister must follow (whether it be from the nine noble virtues or some other Germanic system) of honor, nobility, and not going out of your way to cause undue grief to others.