1)
What is "Panai"?
1.
Rashi citing Targum Yonasan: It is My anger.
2)
Why does it say "l'Ra'ah v'Lo l'Tovah"?
1.
Malbim: Above (18:11), it says "Anochi Yotzer Aleichem Ra'ah v'Choshev Aleichem Machashavah Shuvu Na Ish mi'Darko ha'Ra'ah." The purpose of decreeing evil was so they will repent, and He can give good to them. This was for Yisrael themselves, but Churban was already decreed on the city. Hashem put His face on the city for evil, and not for good. Melech Bavel will burn the city in any case (even if they repent).
