Angry feelings are certainly justified! However, as the disease progresses, it becomes increasingly unproductive to process angry feeling with a person who has dementia. Short-term memory loss and cognitive deficits make it unlikely the person will remember and stick with any agreements made. Beyond being unlikely to lead to longer term resolution, it may make the situation worse.
That’s because people with Alzheimer’s can typically read your emotions until very late in the disease if not the whole time. Their ability to read emotions holds even while they are losing the ability to think logically and communicate verbally.
You absolutely need people and places to vent and process angry feelings. However, over time that will be less and less done directly with the person who has dementia. Introducing anger and negativity is likely the opposite of the calm, peaceful, positive environment you want to create. Especially as the disease progresses, you are the one with the greater capacity to change how you respond, not the person with dementia.