Was there a christian cult before s/paul came along and turned it into the christian church?

Paul was far from the first.

Jesus established His church with Peter, James and John as the Presidency and the other nine apostles with them as the other leaders. They then appointed other leaders to teach and watch over the many local congregations. So, yes, the Church was established before Paul came along. You will recall that Saul was engaged in trying to destroy the Church until the angel confronted him and turned him around into the greatest missionary apostle of the early Church.

I assume so, but we don't know who they were. They were called "The Way." Nothing directly from them survived the passage of time. Some think they were branched off sects of Essenes or Nazarenes. Some think they were Ebionites.

No.

1 Corinthians 15:9 For I am the least of the apostles and do not even deserve to be called an apostle, because I persecuted the church of God.

The church came about on the Day of Pentecost around May 26, 30 AD (Ac 2:47). Paul didn't persecute a "cult", but a church.

Actually you have there the evidence that PROVES what you doubt : For he must have been persecuting based more on the definiteness of HIS belief than on what Christianity taught. But it was JESUS HIMSELF who converted him.