What's With Excommunication Anyway

I really do not understand the Roman Catholic Church policy of excommunicating persons for doing evil.

Does the Bible not teach that we are all sinners. Did Jesus not die on the cross to forgive the sins of believers. Do sinners not need the church to seek forgiveness and redemption.

The answer seems to be that only the most evil are excommunicated.

So, would that include the stepfather who had been raping his nine year old stepdaughter since age six. Apparently not. But it does include the doctors who undertook the necessary medical procedure (abortion) to save her life when she became pregnant with twins as a result of the rapes.

It seems it is the Roman Catholic Church that is the most evil.