Act II, scene 3 might be a good location to find quotes that reflect how Iago intends on manipulating Cassio into getting drunk. Iago's sense of malevolence and ability to prey upon individual weakness is demonstrated in this scene regarding Cassio's weakness for drink. Initially, Iago understands that Cassio is trusting and will act upon that which sounds reasonable. It is in this regard that Iago advises Cassio to drink based on his own "worldly" experience: "I learned it in England where indeed they are most potent in potting. Your Dane, your German, and your swag-bellied Hollander—Drink, ho!—are nothing to your English." This direct quote reflects how Iago is able to encourage Cassio to drink and embrace inebriation.
When Cassio might displays some level of equivocation, Iago recognizes that encouragement and providing a sense of solidarity will allay his fears. Iago affirms friendship in drink in the hopes of manipulating Cassio to drink and put aside any potential thoughts of consequences: "Come, come, good wine is a good familiar creature, if it be well used. Exclaim no more against it. And, good lieutenant, I think you think I love you." It's Shakespeare's genius that he is able to have Iago speak of love and support, qualities that are supposedly everlasting, in the guise of alcohol, the embodiment of temporality and transience. Being able to recognize such a dichotomy and striking within it is where Iago displays pure malevolence in his manipulation of Cassio into drinking.
Finally, Iago directly declares his intentions. It becomes clear that his intentions are part of a larger plan. It reflects his own sense of resentment in desiring to target Cassio and using him as part of a larger configuration that would directly benefit him at Cassio's cost: "If I can fasten but one cup upon him,/With that which he hath drunk to-night already,/He'll be as full of quarrel and offence/ As my young mistress' dog." Iago's plan is directly stated to manipulate Cassio into getting drunk and then being able to control him as a "dog." For Iago, his manipulation of Cassio is reflective of his intense sense of malevolence intrinsic to his being.