It's not entirely correct to say "Taiwan used to be part of China" without context.

The current ruling party has never had any strong claims on it. The old ruling party of mainland China escaped to the Taiwan island after losing a civil war to the other party. The current ruling party of mainland China has adopted imperialistic and nationalistic ideals and has decided the Taiwanese people have no right to be independent of the rest of China and see themselves as the only valid ruler for all Chinese people. All claims on the island itself are just cover for trying to quash independent groups.