Who was the killer?

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

1. The sons of the victim's brother. [ʿAṭiyyah from Ibn ʿAbbās]
2. The sons of his uncle. [Abū Ṣāliḥ from Ibn ʿAbbās] Both these opinions indicate that the killer was more than one person.
3. The son of his brother. [Al-Suddī from his teachers, ʿUbaydah]
4. His brother. [ʿAbd al-Raḥmān ibn Zayd]