In my world mages are powerful and potentially game breaking in a fight. However, I want to limit them to being mostly used by the 'enemy' side, and until very recently it was quite rare for mages to team up to fight large wars.
I generally want to create a world where mages usually are self centred/power-mad/etc. and can't work together - but some decent mages exist. Thus, the act of becoming a mage can't require any evil action. I want to create a system where mages often turn on other mages, such that generally only those willing to fight and potentially kill enemy mages would become mages themselves. However, I'm having trouble fully justifying the level of in-fighting I had planned for them.
A quick overview: mages work by spending Mana to enhance and direct existing power sources. For instance, turning a small candle flame into a fireball and throwing it at their enemy. They generally lack subtlety or the ability to do any long-running magic. They're really good at making things go boom and other obvious, immediate applications of energy/force. Magic is good for little else.
Mages use a Crystal they attuned to to store Mana in. It takes some time to build up mana: a few days to a few weeks to go from 'empty' to 'full' mana reserve, depending on the skill of the mage and how much power their Crystal can store. When a mage uses his full mana reserve he can launch numerous extremely devastating and dangerous attacks, but are quite vulnerable without mana and so need to be careful not to waste mana and keep a sufficient reserve to defend themselves. The power of mages varies greatly with both natural aptitude, experience, and the quality of Crystal used. A mage also can not use a method of enhancing their physical speed/strength others use in this world so as long as they are using a crystal. Thus, a mage is going to be outclassed in any physical battle against soldiers, though their magic is considered far more devastating then any soldier.
I had intended to suggest mages often fight over the highest quality Crystals, to allow larger and more efficient magical reserves, leading to extensive infighting over crystals and often a kill or be killed mentality that comes with getting and holding on to a powerful crystal that other's want. However, I need lower quality Crystals to not be too rare, which would imply mages could exist without being targets of other mages so long as they're willing to settle for not having that powerful a Crystal: basically limiting their maximum supply of mana.
Thus I'm looking for some other options to encourage mages to often fight between each other, to the point that until a recent powerful leader allied them they rarely worked together. I want a world where generally only those with delusions of grandeur or the otherwise power-mad become mages (and/or only that type usually survives to magehood), to encourage a world where generally being a mage only appeals to the less benign individuals, even if the act of being a mage doesn't require any evil actions or intent itself.