Why does the Torah insert the (otherwis superfluous) word "ve'Avad ha'Levi Hu"?
Sifri: To teach us that a Levi is not permitted to withdraw from the Avodah and forfeot his rights to Ma'aser Rishon - because performing the Avodah is an obligation. 1
Oznayim la'Torah: And this explains why the Torah begins the Pasuk in the singular ("ve'Avad ha'Levi Hu") and ends in the plural ("ve'Heim Yis'u Avonam") - because it is only an individual Levi who might withdraw (See Oznayim la'Torah DH 've'Avad ha'Levi Hu' #1).
Why does the Torah begin the Pasuk in the singular ("ve'Avad ha'Levi Hu") and ends in the plural ("ve'Heim Yis'u Avonam")?
Oznayim la'Torah #1: Refer to 18:23:0.1:1*.
Oznayim la'Torah #2: To warn the Levi'im that, even if there are a large number of Levi'im and it seems that his services are not realy needed, he is not permitted to withdraw even from one Avodah - as if he was the only Levi. 1
See Oznayim la'Torah DH 've'Avad ha'Levi Hu' #2.
What is the meaning of "ve'Heim Yis'u Avonam"?
Rashi: It means that the Levi'im will bear the sins of Yisrael, since they are responsible to warn the Zarim to keep away from them (when they serve in the Mishkan).
