Paul acknowledged that it's possible to preach Christ with the wrong motivations. Some people were jealous of Paul's popularity among Christians, so they mimicked his message to become more popular. This was not a common motivator based on the persecution Paul and others suffered for their faith. Others saw Paul as a rival or competition and sought to become more widespread through their preaching outreach. It's also possible that those who hated Paul's message deliberately spread it to inflame his enemies.
In verse 18, Paul notes that a person's motivations for spreading the gospel don't change the fact that the gospel is being spread. As long as what these envious, jealous people said was the truth, it made little difference why they said it.
In addition to these two inappropriate motivations, Paul noted that some who preached Christ in Rome did so "from goodwill." They had an appropriate, godly motivation to share Christ and to help lost people be found in Jesus. By this time, in the early AD 60s, many shared Jesus in Rome. Before long, the city would be considered the capital of Western Christianity.