Why the marquis was mean/abuse Shuri ???

possibly dementia/alzheimers/stg like that. the marquis knew he was ill and deteriorating

(but even without the illness he probs wasn't all that nice of a man, just less unreasonable)

Correct me if I wrong, but I think he started to get mean/abuse because of that necklace and letter (I forgot who gave it) but I think that was the start of his insanity.

The Prince give her the necklace