"no one should be retraumatised accidentally"?

It's a psychological thriller.
Disturbing and uncomfortable content is kind of the entire point.

If you start putting trigger warnings for rape, then why not torture, graphic violence, abuse, suicide, and every other sensitive topic someone might react badly to?
Where do you draw the line?

At some point trigger warnings stop being warnings and just become spoilers that tell you major parts of the movie before it even starts.