Defiling should be really tempting - both over the long term and in the moment. It should be tempting to start, and harder and harder to stop.
In the lore once a Wizard defiled they would struggle to go back to harmony with the land. In the same way that an alcoholic can’t have just one drink’. Preserved and Defiler was a subtype, kit or character option. It’s a bit like Fallen Palladin. Redemption should be possible but would require effort.
Idea: once you use a spell slot to defile,
that slot can ever after only be used to defile.
For example, you've got three 2nd-level spell slots, and you use one to cast
scorching ray with a defiling effect. The spell gets a power boost, plants die, everyone hates you now, etc. But also, you now have two 2nd-level normal spell slots, and one 2nd-level spell slot that you must use with a defiling effect (spell gets a boost, plants die, everyone hates you more, etc.). If you have a particularly nasty encounter and feel the need to power boost your
scorching ray twice, well, now you are down to one 2nd-level normal spell slot, and two 2nd-level defiling spell slots.
Obviously this doesn't work at all for warlocks (but honestly I think warlocks would be Templars in the setting -- Pact of the Dragon King). And maybe there's a way to cleanse your spell slots; like each time you level up, or spend a year without defiling, you can turn one defiling spell slot back into a normal one.