He is mentioned very few times in our sources; we do not have his 'biography'; and the statement attributed to him in our present mishnah is the only statement attributed to him at all.
2:
This should not surprise us too much, because in these comparatively early days of the development of the oral tradition most of the sages were completely anonymous. In fact, the first one to be mentioned by name was Simon the Just (beginning of 2nd century BCE) and from his time on so far only four other sages have been mentioned by name in more than one hundred years. Nittai ha-Arbeli is the fifth.
3:
As we have already seen, the times in which Nittai was active were dangerous times. The flagrant manner in which Alexander Yannai espoused the Sadducean cause was a continual source of civic and social friction, as we saw in Avot033, Explanation 11. This social unrest is probably the background to Nittai's teaching. In all probability the "bad neighbour" from whom he advises we distance ourselves is the neighbouring Sadducee. When we consider the Sadducees and the Pharisees as movements we tend to forget that socially they were completely mixed. In his elaboration on our present mishnah Rabbi Natan [Avot de-Rabbi Natan 9:1] indicates that the distancing is social and not physical:
Keep your distance from a bad neighbour, whether he lives in the same building or in another or whether [you meet him] in the countryside.
It will be easier to understand this comment by referring back to the description given in
Avot028, explanation 3.
4:
Neither Rambam nor Rabbi Ovadyah of Bertinoro seem to be aware of the historical background to this teaching, and they give it a moralizing slant: the more one associates with wicked people the greater the likelihood that one will learn from their wicked ways. While psychologically this is probably a valid assumption I have always been wary of this kind of reasoning: why should we always assume that the righteous will be corrupted by the wicked? Surely there is also a chance that the good people might influence the others, and if they always keep themselves in their own little "ghetto" what positive influence can they have?
5:
The last item in Nittai's teaching is clearly linked to the doctrine of Resurrection. As we have seen this teaching was a new teaching, introduced into Pharisaic circles around the time of the persecutions which preceded the Maccabean uprising in 165 BCE. You will recall that the Sadducees rejected this teaching outright as having no Torah basis whatsoever, and the Pharisaic scholars turned hermeneutic cartwheels in order to give the new teaching a very flimsy basis in the Torah. [See Avot022 for a full explanation.]
6:
It seems that this new teaching was not entirely satisfying for the proverbial "man in the street": when suffering at the hands of the wicked it may be intellectually comforting for him to believe that at the end of history, at the great day of judgement, he himself will be ultimately vindicated and his oppressors justly condemned. But in the meantime he is suffering under this oppression and the idea of ultimate retribution in another life is rather cold comfort. Thus the initial enthusiasm with which the faithful had received the new teaching gradually cooled, and Nittai has nothing left in his arsenal except the pious exhortation that one must never lose faith in the teaching of "Teĥiyyat ha-Metim", the resurrection of the dead at the end of time.
7:
Nittai ha-Arbeli brings to an end the early period of anonymous and semi-anonymous sages. From the next generation onwards the leaders have a more substantial biography.
