We can take the proscriptions against these spells as sound Game Mastering advice:
- The Imperius Curse - At it's heart, the reason why the Imperius Curse is regarded as unforgivable is because it robs its target of their will and agency. As a Game Master, you should be careful not to rob your players of the agency to make decisions about their characters. Outside of brief effects (charm person, truth serum, fear and sanity rules), the player should be making the majority of decisions about what their characters do and how they react to the setting of the game. It's the players' job to pull their characters' strings, not yours. This goes double for putting the characters in situations where they don't have agency, can't make real decisions about the outcome of the game, and can't really affect the direction the game goes in.
- The Cruciato Curse - You should also be careful about torturing your players. (But maybe not their characters; as a GM your job is to put the characters in tough spots and to get the players to make difficult choices about them.) How do you torture the players? Long-winded soliloquys from the game's villains, info-dumping quest givers with too much to say, setting descriptions that slow the game to a crawl, stopping play for a Talmudic study of the rule book when a question about game mechanisms comes us, etc. As a GM, you probably have more control over the pace of the game than your players do; keep things moving, and resist the temptation to bog the game down with your beautiful prose poetry, scripted NPC reactions, and desire to get the rules perfectly "right." It's fine to dwell on a particularly fine moment of play, but be conscious of the amount of player buy-in you're getting and move on when you see it fading.
- The Killing Curse - In most games, character death is on the table to provide the utmost extremity of the tension that stems from danger. (Player death, on the other hand, is still illegal in most localities.) However, although the threat of death can create great moments of tension, that doesn't mean it's the most interesting option at your disposal as a Game Master; often, killing a character is the least interesting thing you can do to them. Instead of putting the characters in situations where death is the most obvious outcome, consider how you might put them in situations that ramp up tension by making their lives more difficult or by putting them in situations where the possible outcomes feel worse than death--and are therefore richer in drama. Endangering beloved NPCs, having the characters' goals slip away from them, injuries that are debilitating rather than fatal, cruel reversals of fortune, etc. are all useful things that complicate the players' relationship to the game and their characters' place in it, and they're also often far more interesting than the bland finality of death. That said, if they gotta die, they gotta die.