When dealing with acidity, I tell my students 3 words,,,,,EXAMINE THE ANION. The halogens do indeed increase acidity because they can remove electron density from the anion, and make it less negative. This will translate into making it more stable, thus the compound is more acidic. A stable anion more readily forms !!! When dealing with directing influences on a benzene ring, we do indeed get some electron withdrawal from a halogen , but resonance competes. Two factors are at work. The inductive effect operates to raise the energy barrier of the transition state, thus the reaction is a bit slower.....BUT.......resonance effects help stabilize the intermediate in the ortho and para pathway to a greater extent than the meta pathway. An extra resonance form is produced. Thus , a halogen does deactivate a molecule somewhat, BUT since an additional resonance form is seen in the ortho-para pathway, we see the major product distribution as ortho-para . . Another group that you should know is called the nitroso group.....NO. This group works in an analogous fashion to the halogens. If you are curious,,,,,,a Fluorobenzene is about as reactive as benzene alone. Why ? Not such an easy answer, but although electron withdrawing, the overlap of a carbon 2p orbital with a fluorine 2p orbital is similar in size, and very energetically favorable ! Not so with the large 5p orbital on iodine !!!!
