What is meant by their spreading mischief on earth?

Imām Ibn al-Jawzī (d. 597 AH / 1201 CE) summarized the scholars' opinions in his book "Zād al-Masīr":

1. They invite people to disbelief. [Ibn ʿAbbās]
2. They engage in sinful acts. [Al-Suddī, Muqātil]
3. They block the path for those who seek to migrate to the Prophet ﷺ, in order to prevent people from embracing Islam.