How do you fix a kettle that makes the boiled water taste bad?

Perhaps you can try descaling your kettle. You can either purchase a descaler or put 1/3 white vinegar + 2/3 water in the kettle and boil that. Leave the solution in the kettle overnight after boiling. The next day, dump out the vinegar solution and boil with just water. Leave that overnight and rinse out the kettle and you can use the kettle for making hot beverages now.