My bathroom has a water leak in the wall. Black mold continues to grow back after treatment with bleach products or kilz. Isn't this a health hazard?

YES!!! Absolutely. Mold is a big problem and source of respiratory symptoms, allergies, and asthma. In many cases, the whole area effected must be removed and replaced. If may cost more now, put is will pay off in prevention of health complications in the future.
No doubt. You need to have a professional company come in to remediate this problem immediately for your short term and long term health.
Address the leak. Indoor mold growth requires excessive moisture. Until the water issue is fixed, mold growth will recur even after bleaching or killz. Such molds cause respiratory issues, primarily: cough, sinusitits, asthma, etc. There's more hype than science about indoor molds at present.