People don't often realize how subtle changes in language can change our thought process. It's just how human brains work sometimes.
The old bit about smoking and praying is a great example. If you ask a priest if it's alright to smoke when you pray, they're likely to say no, as your focus should be on your prayers and not your cigarette. But if you ask a priest if it's alright to pray while you're smoking, they'd probably say yes, as you should feel free to pray to God whenever you need...
Now, make a machine that's designed to be agreeable, relatable, and make persuasive arguments but that can't separate fact from fiction, can't reason, has no way of intuiting it's user's mental state beyond checking for certain language parameters, and can't know if the user is actually following it's suggestions with physical actions or is just asking for the next step in a hypothetical process. Then make the machine try to keep people talking for as long as possible...
You get one answer that leads you a set direction, then another, then another... It snowballs a bit as you get deeper in. Maybe something shocks you out of it, maybe the machine sucks you back in. The descent probably isn't a steady downhill slope, it rolls up and down from reality to delusion a few times before going down sharply.
Are we surprised some people's thought processes and decision making might turn extreme when exposed to this? The only question is how many people will be effected and to what degree.
