My world is similar to modern-day Earth, though with additional magical realms not readily accessible from the Rational Realms that are familiar to us. In this world, as many as ten people in every million have the necessary gift to become magi. To become a magus requires training from an early age, from around 5-6 years old, and training goes on for 12 to 15 years before the apprentice or student is considered to be a magus.
A newly graduated magus is not particularly powerful. If they had specialised in combat, they might be a match for a platoon of modern soldiers. However, an arch-magus, a magus who may have studied most of their life, could concievably be a nuclear-level threat if they specialised in combat.
Why haven't there been magical disasters? Well, there have been. Our world is broken into the rational realm and the many magical realms because two nations with powerful magicians went to war and the magics they used literally broke the world into many pieces. Why haven't there been more disasters since? That would be because in the 'old days', since the world was broken, the magi who trained apprentices used to look out for signs of instability in their apprentices, and if such signs appeared, the apprentice would not become a magus. If they were a sufficiently young apprentice without much power, they might 'fail' their apprenticeship, be told that their gift was somehow incomplete, and they couldn't be trained any further, and then abandoned, and possibly magically maimed so that they could no longer do magic. Other, more advanced apprentices who were found to be future monsters might simply be killed. In the old days, an apprentice had the same (lack of) rights as a chattel slave, and their master could do whatever they wanted to them, including killing them, on the understanding that the apprentice might eventually become a magus themselves and may wish to seek revenge.
Magi who complete their apprenticeships are made to join the brotherhood of magi, and swear oaths to, in essence, act in reasonably moral ways, primarily so as to not inconvenience their fellow magi or bring magi in general into disrepute, on pain of disciplinary action ranging from fines of magical resources, up to and including death for breaches of their code of conduct. The code of conduct includes a clause requiring magi who teach to ensure that their apprentice both does and will follow the code.
In the last 20 years, new discoveries in the arts of magic have been spreading through the world, and some of these new-model magi have begun teaching their own students. They have established a school where the gifted students are taught in a far more systematic manner, where the students are much less likely to suffer the abuses of the old apprenticeship system. The students there are treated with dignity, and are not supposed to be summarily crippled or killed on the whim of a sole master as once might have happened.
The new-model magi are quite capable of determining if a student is developing a personality that is likely to lead to their becoming a monster post-graduation (or even before), and these teachers are capable of rendering a student unable to perform magic, killing them, or re-writing their minds so that they would no longer be inclined to be the monster it is suspected that they might become.
My problem is that these teachers are using a modern 20th/21st-century upbringing and morality to judge their students and deal with the problems their training might cause.
Using a modern western nations morality, how might these 'future monster' students best be dealt with? It doesn't seem that there are any good solutions, when there are no certainties that a potential monster really will become a monster, and the means of ensuring that they probably won't become one are invasive, and if certain, are mentally crippling or fatal.
Since this is a question of the ethics of our own society (or one very similar to it), and WB SE users can be considered to be representative of our society, the most upvoted answer should be the one that would be considered to be the most acceptable - or the least unacceptable - to the greatest number of people in our society