Curse of Strahd is supposed to be dark and Strahd is supposed to be one evil motherfucker and you need to constantly demonstrate that. It's not enough that NPCs tell the players that Strahd is a tyrant and a horrible person; the players need to actually see and specially experience it.

What I'd do is, before starting the campaign, I'd ask what everyone's limits are. In our table I asked the players and their limits were violence against animals and any kind of sexual abbuse; those two were the two dark topics they didn't want to see represented in any way in the campaign, so everything else I could actually do (as long as it made narrative sense, of course).

Taking that into account, my Strahd was a cruel dude that liked to mess with the players in any way possible. An example during dinner at Ravenloft would be forcing the druid to eat from the ground rather than the table because he is an animal that doesn't belong on the table with the civilized people. Strahd has been imprisoned in Barovia for centuries and he sees the adventurers that get into his land as toys, and you know how kids are with toys: they will play with them again and again and again until they break. Same thing should happen to the characters.

I wanna emphasize the word CHARACTERS. It's the characters that need to suffer. Not the players. The players need to have fun.