Numbers Rabbah 19:23
"The people came to Moses and said, 'We sinned by speaking against the Lord and against you." (Numbers 21:7) They realized that they had spoken against Moses and they prostrated themselves before him and said: "Intercede with the Lord to take away the serpents from us! . . . And Moses interceded for the people." (ibid) This serves to acquaint you with the meekness of Moses who did not hesitate to beg mercy for them, and to acquaint you also with the power of repentance, for as soon as they said, "We have sinned," he was immediately reconciled to them, for he that is in a position to forgive should not be cruel.
This far into Numbers, we are inured to the Israelites' complaints. The complaint of Numbers 21 takes place in five quick verses and stands out more for the unusual bit about the snakes than it does for the fact or content of the Israelites' gripe. But pause for a moment to feel the full drama: the people whine that Moses "made them" leave Egypt and complain about the "miserable" food. God punishes them with a spate of serpents whose biting is fatal to many. Castigated, the Israelites approach Moses and apologize ("We have sinned!"). Moses, without a word of reproach, it seems, intercedes and God grants a cure for the snakebites.
From the soap opera tension of the story, the midrash draws a weighty moral lesson. "He that is in a position to forgive should not be cruel." Moses would have been well within his rights to berate the people who have so consistently mistrusted and mistreated him. Rather than respond with anger, however, he takes them seriously, respecting their request, and perhaps even giving them the benefit of the doubt regarding their sincerity.
The power of repentance is strong, and as the lesson of Yom Kippur teaches annually, anyone is capable of achieving forgiveness through sincere teshuvah. Of equal importance however is the lesson about being one who forgives. It is so hard to withhold judgment, to check one's anger and respond with a "meek" heart. Asking forgiveness is hard, but granting forgiveness can be harder. We would do well to emulate Moses's example.