Why did he tell them not to fear?
Radak: If you will not veer from following Hashem, you need not fear.
Malbim: Had Hashem become angry with you for no reason, it would be proper to fear that His desire for you changed from good to bad. However, the change is due to your evil deeds. Therefore, do not fear, just do not veer from following Hashem, and He will return to rejoice over you, for good!
What is "Kol ha'Ra'ah ha'Zos"?
Radak: It is requesting a king.
Radak citing Targum Yonasan: [You caused (some texts - did)] this bad matter (the rain).
What is serving Hashem with all your hearts?
Malbim: It is amidst love, due to His exaltedness, without motivation of reward or avoiding punishment. Even if you thought that you can benefit from serving something else, you should refrain.
