Pashas Ki Tetzeh includes one of the more mysterious commands, that of the execution of the rebellious son בֵּן סוֹרֵר וּמוֹרֶה As Rashi explains, he is killed for his end [as he will come to commit violent crimes even though he not committed them yet]. This raises the problem of how to reconcile a punishment based on a prediction with what we see elsewhere in the Torah?
Accordingly, my grandfather cites the Sifsey Chachamim's question of how this fits with what we learn out about Yishamel about whom it say ba'asher hu sham (Berishis 21: 17) on which Rashi says that he ws judged according to his deeds at the time rather than what he [or his descendants] would be doing in the future. However, in the case of Yishmael, he had not at that point doine anything that indicated a connection to a future intention of killing the children of Israel through thirst. And the Heavenly court does not judge according to one's future. On the other hand, there is a mitzvah on the earthly court to judge the wicked according to their futures.
My grandfather finds that the suggestion that the earthly court has to be harsher than the Heavenly one fits with what the Tosfos say in Brachos 7a on the account of R' Yeshoshua ben Levi's foiled attempt to curse the Sadduccee. The conclusion was that it was wrong to try to harm him because of injunctions for mercy. They include It is written: "And His tender mercies are over all His works." And it is further written:"It is not good for the righteous to punish."
Tosfos comment that even though the rule of moridin velo ma'alin applies to Sadduccees and the mosrin, that's only for human judges. That does not apply for death in the hands of Heaven.
My grandfather says there appear to be some difficulties with this because many sins are punished by Heaven if not punishable in court. Consequently, much study is necessary to truly understand the Tosfos. The editorial note at this point reminds us of the fact that the Heavenly court only punishes someone above age 20, which would indicate a kind of leniency over the earthly one.
My grandfather then suggests that the particular situation described for R' Yehoshua is one in which he felt personally vexed by the Sadduccee. In such a situation, when it is a private wrong, the greater virtue is to forgive. My grandfather explains that we'd have to assume that there was no public harrassment, for it there were, it would be forbidden for a Talmid Chacham to allow an attack on his honor for which he is enjoined to be nokem venother kenachash (Yoma 22, Yoreh Deah 243). There is a distinction here between private and public action. It is laudable to forgive private vexation. But if the Sadduccee poses a threat of infecting others with heretical inclinations, then the directive is moridin velo ma'alin -- not out of motivation for one's individual honor but for the honor of Heaven and to remove a public danger.
Related post: http://kallahmagazine.blogspot.com/2015/10/dvar-yehudah-parsha-points-from-my.html