What is so understandable? And as for Serbian animosity towards Croats, it's understandable considering what happened in WWII. Who came first, the egg or the chicken?
Croatia declared independence from Yugoslavia and was willing to proceed with it by using military force against the political and military institutions of the SFRY. In response to Croatia's declaration of independence, the Republic of Serbian Krajina declared independence from Croatia and was willing to proceed with it by using military force against the political and military institutions of the Republic of Croatia.
You see, this attitude of the Republic of Croatia is exactly what caused the war in the first place and made a political solution almost impossible. At the same time the leadership in Zagreb was demanding certain rights for their nation and refusing to allow these same rights to the Serbian nation.