It's understandable, but it's not the way a native speaker would normally say it.
I'm reluctant to call it wrong. Maybe someone else will comment.
I agree that "were deeply hurt when seeing" is not good and wouldn't be said by native speakers. That grammar suggests that they were already deeply hurt before they saw the torchbearer being attacked.
Raymott's two suggestions are much better, and 'upon seeing' is also used.