Has anyone heard of pseudodementia?

Is it possible that the Lexapro just kicked in? Two months is about what it takes for many antidepressants to start having an effect.

Perhaps depression was causing her hateful attitude?

There are often times and situations in which both those with dementia and mental illness CAN and DO understand limits. I think not in a way we do, but with a kind of memory of fear in that "Oh, oh--this resulted in that once, so..." Almost as a child does, without real understanding or though. Explosions of rage and anger often have the same result on them as they have on toddlers, in that for a while the toddler remembers there was an explosion. No real understanding of why or what to do about it, but momentarily stunned to silence. I used to float to psych and when there was a lot of acting out around the desk we could actually say "You can't be doing that or you will be confined to room" and they could stop themselves quite readily. So were in more "control" of their actions for at least a while. It is hard to judge how the mind of someone who computes things differently works or doesn't work; it's a mystery. Read everything you can ever find by Oliver Sacks about how our minds work. How the mind of those with afflicted minds work. It was his life's work.