So Markc reminded me (probably unintentionally) that I should take a look at what RMU actually says about this. There is an interesting suggestion in the Customizing Magic section of Spell Law (2.8):
One method to handle the Evil spell lists is to treat them as morally corrupting in the manner that follows. Each of the Evil lists is linked to a specific vice: greed, cowardice, disloyalty, spite, etc. Each list might be constantly tied to a particular vice: Darkness to dishonesty, Essence Twisting to arrogance, Disease to slovenliness, and so on. Alternatively, the associations might vary from individual to individual, with Faceless Jack becoming vain from his studies of Demonic Pacts, while his rival Mary Malice suffers gluttony as a result of learning that same list. Whenever an opportunity arises to express one of the vices that the character has as a result of delving into the Dark Arts, but the player of that character (or the GM on behalf of an NPC) wishes to declare some other action that does not express that vice, the character must make an RR (modified only by SD bonus and, possibly, circumstances) against the corrupting effect of the relevant list (treat ranks in the list as the level of the "attack") or act according to that vice instead.
So, rather than becoming increasingly controllable, you and the player could pick a vice that is related to the spell list, and they will become more and more prone to acting accordingly. In the case of the Wounding list, which is all about causing physical harm, an obvious choice might be sadism, but depending on where you feel like the power of evil magic comes from, it could be any quality that fits that source. And you could use NPCs who are more advanced in that magic as examples of what people are like when they go down that road.
That has a lot of roleplay potential, it doesn't make the character unplayable, and it can come up at any time rather than being dependent on encountering evil entities.