Why did the church deal so cruelly with heretics?

The church dealt with heretics very harshly, demonstratively and instantly. This can be explained very simply, because many church teachings and generally accepted concepts of that time simply could not be explained by science, and some even by the same science could easily be refuted. So, it was impossible to allow the victory of science over the church. In addition, everything that was not associated with the church was considered from Satan and had to be instantly destroyed and eradicated. Heretics could seriously undermine the influence of the church in society.