Netziv (Introduction to Bereishis): Bereishis was called Sefer HaYoshor (the Book of the Upright) by the prophets. This is because it is about the Avos—Avraham, Yitzchok and Yaakov who were called Upright by Bilaam. We need to understand why he singled them out by the title Upright instead of other descriptions such as tzadik or chasid? Furthermore, why is Bereishis singled out with this description of Upright? Another question is why Bilaam asked that his end should be like those who have such a description? The praise of Uprightness is to reinforce G-d’s judgment in destroying the Second Temple which as a generation of tzadikim and chassidim and those devoted to learning Torah—however they were not Upright in the mundane world. Therefore, because of the baseless hatred in their hearts to each other they suspected that whomever they saw who did not conduct himself according to their opinion in Yiras Shamayim—that he must be a heretic. Consequently, this led to much killing and all the evils in the world until the Temple was destroyed. Thus, there was an acknowledgement of the Uprightness of G-d’s judgment in that He would not tolerate tzadikim like these. Rather he wanted tzadikim who were upright in the world. Because even if the non-upright tzadikim were motivated by religious consideration—such conduct destroys the world. Therefore, this was the praise of the Avos that besides being tzadikim and chassidim and lovers of G-d to the ultimate degree they were also upright. That means that they conducted themselves in relation to the peoples of the world—even the debased idol worshippers with love and were concerned about their welfare in regards to the preservation of Creation. This we see in the pleading of Avraham for the people of Sedom—even though he had total hatred for them because of their wickedness—nevertheless he wanted them to live
Netziv (Devarim 32:5): It is known that the destruction of the First Temple was because of idolatry, sexual immorality and murder which angered G‑d. The destruction of the Second Temple was because of sinas chinom. We have explained in Bamidbar (35:34) that they were involved in the study of Torah and serving G‑d nevertheless most of the killing was done for the sake of G‑d. In other words they would label as a Sadducee (heretic) anyone who transgressed any of the commandments. They would then have them punished as a heretic and thus their conduct was very destructive – even though they thought they were motivated by concern for the honor of heaven. … Thus the problem of the Second Temple was that ostensibly good deeds were now mixed with bad motivation. Thus it was hard to separate the good from the bad since the bad was done for the sake of heaven.