Baba Basra 109b - 110a - Understanding Yehonasan (Grandson of Moshe Rabbeinu)

The gemara 109b says that a persons children will inherit the attributes of the family he marries into. Moshe married the daughter of Yisro who spent his life steeped in idolatry and therefore had Yehonasan as a grandson who served as a priest for the idol of Micah. But, Aharon who married the daugher of Aminadav (sister of Nachshon), had Pinchas who was a great tzadik.

The connection between marrying the sister of Nachshon and having a child like Pinchas seems clear. Nachshon was known for his mesiras nefesh to be mekadesh shem shamayim, being the first to jump into the Yam Suf. Pinchas was also moser nefesh by being mekadesh shem shamayim by killing the Nasi of Shimon, despite the ridicule he had to suffer (as the gemara says that all the shevatim made fun of him that he descended from Yisro who worshiped Avoda Zara).

It would seem that the connection between Yisro and Yehonasan is also clear. They were both involved in idolatry. But, the gemara on 109a explains that Yehonasan was misled by a tradition that he heard - "A person should alwasy rent himself out to avoda zara, rather than take tzedaka". Yehonasan understood this literally, that for parnassah one may work as a priest for avoda zarah. The Rashbam seems troubled by how he could made such a mistake and writes - אלא שלא יהא לבו לעבודה זרה. Yehonasan thought that as long as he is not intending to worship the avoda zara, he is not doing anything wrong. This seems to be an honest mistake based on the misunderstanding of the tradition (that avoda zara meant menial tasks, rather than actual avoda zara - see rashbam who says a big chiddush :) - לעשות מלאכה להתפרנס אין כאן גנאי), so why does the gemara indicate that he was a Rasha - he never actually worshiped idoaltry?

The Rashbam quotes a Yerushalmi that elaborates on the behavior of Yehonasan. Yehonasan was entirely motivated by $. People would bring sacrifices to the Pesel of Michah, to which Yehonasan would say that they are wasting their time. He would then tell them that they should give him precious gifts and he will bring it to the avoda zara. When they left he would indulge in the gifts. When confronted he admitted that the Avoda Zara has no power and he is only working there for parnasah. The aveirah of Yehonasan is that his desire for wealth blinded him from realizing what he is doing. He may have honestly been confused and thought that the tradition that he had permitted what he was doing, but the only reason he made such a grave error is because he was blinded by his desire for wealth.

Where did this great desire for wealth and physical possessions come from? Perhaps this came from Yisro. In parshas Yisro we find that Yisro comes (according to Ramban it was prior to matan torah), then after giving moshe advice he returns to his family, but the Ramban explains that he came back again while the Jews were still camped as Sinai. Then in parshas B'ha'aloscha (10:29) he tries to leave again. Why? Rashi explains - אם בשביל נכסי אם בשביל משפחתי. He wanted to go back to his wealth rather than join the Jews into EY. Moshe then begs him not to leave because as rashi explains, people will say he only converted to get a portion in EY, so when he realized that converts aren't entitled to a portion he left. Moshe then has to guarantee Yisro some financial incentive to get him to stay - והיה כי תלך עמנו והיה הטוב ההוא אשר ייטיב ה' עמנו והטבנו לך. Rashi explains that the "good" that is being referred to is that when EY was divided, Yisro received "doshna shel yericho", which he would have until the time when the Beis Hamikdash would be constructed. The Ramban (yisro) understands from this rashi that moshe successfully convinced yisro to stay by offering financial incentive. Targum Yonasan also says that the good that Moshe promised to Yisro was בפילוג ארעא, namely a portion in EY. Based on this, we can suggest that the poison that Yisro brought into the genetic pool of Moshe's descendants was not service of Avoda Zara because Yehonasan his grandson never actually worshiped avoda zara (also, Yisro did teshuva from avoda zara). Rather, the poison that Yisro brought which the gemara refers to, is the great desire for material wealth that caused Yehonasan to make such a fatal error.