How did the Romans view the Christians and why did they persecute them?

How did the Romans see the early Christians? In particular, what was the attitude of the Roman state towards them? Famously, there were persecutions, first under Nero and then at various times and various places after this. The Romans rarely attacked other reliions. Instead, most were absorbed, often given a Greco-Roman veneer. Why were the rare exceptions, e.g. the cult of Bacchus in 186 BC, the Druids under the Julio-Claudians, singled out for repression by the empire? Today we try to see everything from the point of view of the emperor and his governors, and also local leaders, which means concentrating on the Roman sources. Until the middle of the third century AD, there were no empire wide persecutions, so just what was going on before?