In The Merit of Yisroel Saba:
The Bnei Yissochor zy”a (Maamarei Hashabasos, Maar 6, Ois 12) explains this pasuk by saying that Hashem is so merciful that He provides goodness even for those who are undeserving. It is true that “all His ways are with justice” but He still provides for the unworthy by first sending goodness to those who are worthy and then allowing those who do not deserve it to benefit by being in proximity to them.
He uses this idea to explain the pasuk (Bereishis 12:29) that says that when Hashem destroyed Sodom “He remembered Avrohom and He sent out Lot.” Why is Avrohom mentioned in this pasuk? Because when He decreed that Sodom must be destroyed, Lot had no merits to make him deserving of being saved, especially since he chose to live amongst wicked people. Hashem wanted to rescue him, but there was no way to do so in a fair and just manner.
So what did Hashem do? He sent the Malach Rafael to heal Avrohom, which certainly was a fair thing to do. Part of healing Avrohom was mending his broken heart. Since he was very anguished over Lot’s plight, Lot had to be saved in order to heal Avrohom. Thus, Hashem found a just way to save Lot.
In this vein, the pasuk is saying that Moshe told Yisro to come with the nation and it would be good for him. He told him not to worry that he was undeserving, as Hashem has spoken good about “Yisroel”, which is a reference to “Yisroel Saba”, i.e., Yaakov Avinu. Moshe was referring to Hashem’s promise to take care of Yaakov, which includes caring for his offspring, even if they are unworthy.
