Malbim: The Shelichus was not for the sake of Ninveh, to make them repent. Only in Yisrael, Hashem sends Nevi'im to make people repent ? "Ki ha'Goyim ha'Eleh Asher Atah Yoresh Osam El Me'onenim v'El Kosemim Yishma'u Lo Chen Nasan Lecha Hashem Elokecha; Navi mi'Kirbecha me'Achecha Kamoni Yakim Lecha" (Devarim 30:11-12)! It was for the sake of Yisrael. Ashur was prepared to be the staff of Hashem's anger to subjugate Yisrael, who were liable. Hashem wanted Ashur to repent, so they could fulfill His decree. Also people should not complain that a Rasha devours one more virtuous than himself ¹ (Chabakuk 1:13). Hashem wanted to show that Ashur has more merit than Yisrael ? they heeded the Navi and repented, but Yisrael did not.

Rashi: What I tell him to say.

Radak: It does not say here, but we infer from what he called in the end "Ninveh Nehpaches" (3:4).

Malbim: It is not what he said in the end. The decree was not yet sealed. Had Hashem commanded him to tell a Nevu'ah, he would not have fled ? one who withholds his Nevu'ah is Chayav Misah! Rather, Hashem merely commanded him to speak Musar and rebuke them to repent. Yonah thought that he is not Chayav Misah for refusing; he merely transgresses the Aseh that Hashem commanded him to rebuke.

Radak: Hashem supervises even over other nations when their evil is great in theft. The same applies to Dor ha'Mabul and Anshei Sedom "Ki Mal'ah ha'Aretz Chamas" (Bereishis 6:13). Theft ruins society. Hashem wants the world to be settled! Other Aveiros are not important in front of Him to supervise over them ¹ , except for Yisrael ? "Rak Eschem Yadati mi'Kol Mishpechos ha'Adamah" (Amos 3:2), like I explained there.

Malbim: When the sin is great, it prosecutes in front of Hashem. After the decree, it descends. We infer that no decree was sealed yet.