Mercule
Adventurer
I've read I, Strahd, but I remember very few of the details, anymore. I haven't read any other Ravenloft novels.But you do raise an interesting point. I wonder how many of us who truly, deeply dislike this change are also the ones who consider the Ravenloft novels an important part of "our" canon. The novel I, Strahd is easily one of the best of the line, and is also the source of a lot of the deeper looks into Strahd's background and personality. It heavily influences my view of the character, and indeed Barovia as a whole. I can quite easily see how those who haven't read it might have a different view of the importance of specific details in his history.
I consider Tatyana important because ripping a countryside from its home plane is not something that happens gradually. There must be a catalyzing act that does that. It can't be "just one more step".
In murdering Sergei, Strahd willingly casts off the last of his humanity to get what he (thinks he) wants. The cursed part is because he didn't actually get what he wanted -- someone to see him as human and alive as Sergei. Instead, he threw away his only hope at that. While Strahd was a very bad man, he was "tricked" into a bad deal (poorly worded wish? deal with the devil?).
That doesn't mean you should feel sorry for Strahd. Even though I say "tricked", it wasn't like he didn't know killing his brother was wrong. He's also smart enough to do the math that killing a woman's beloved on her wedding day is pretty far down on the list of "ways to win a soulmate". He made his bed and now must lie in it.
Tatyana and Sergei are important because they represent what Strahd sought. Without them, he would have been just another harsh despot. It was his envy that set him down that path and it was his ruthless tactics that drove happiness away; he couldn't let go of the conqueror's approach.