What era are we talking about? World War II?

Yes, Croatian nationalists murdered Serbian civilians as part of a (albeit anarchically applied) top-down policy in order to engineer a Catholic nation state. I tackled this in a fair bit of detail in an answer here.

In the Balkan Wars of the 1990s?

Yes, Croatian nationalists murdered Serbian civilians. Which isn't to say other parties didn't also murder civilians.

If you focus your question I'm sure someone will be able to give you a more considered answer - we could be talking about the alleged behaviour Croatian K.n.K units on the Serbian front, for all I know!