When Moshe ascended Har Sinai to receive the Luchot, Hashem was writing the words Erech Apayim, “slow to anger.” Moshe asked Him if this was referring to patience and mercy regarding a Tzaddik when he sins. Hashem responded that not only did it refer to patience with the Tzaddikim but with Reshaim as well. At the time, Moshe did not see the wisdom in being forbearing with Reshaim, but Hashem assured him that one day this trait would be helpful to Bnai Yisrael.
When Bnai Yisrael sinned by listening to and believing the spies’ evil report, they fell into the category of Reshaim due to their constant sinning and complaining. Upon hearing that the nation was to be annihilated, Moshe pleaded with Hashem for their salvation. Recalling the above conversation, Hashem said that he would act according to Moshe's words and be patient only with Tzaddikim. Moshe then reminded Hashem that He promised to be calmer with Reshaim as well and backed up this claim by using a line from Pirkei Avot that the mighty people are those who control their emotions. Moshe’s appeal worked, and the punishment that was to be handed to Bnai Yisrael was dissipated and meted out to last through the years in the desert and for future generations as well.
As this dialogue supports, the attribute of being “slow to anger” is an important one. Although people may seem to be Reshaim now, there is always a chance for them to repent and return to the path of righteousness. By being patient when they sin and by calmingly offering them advice on how they can improve themselves, we give them a new hope that they can eventually become Tzaddikim, ones who can merit the building of the Bait Hamikdash, just like the Bnai Yisrael did even after they sinned so grievously.