For there were many in the congregation that were not
Or purified from uncleanness, contracted either by idolatry, or through such things which, according to the ceremonial law, made them unclean, and from which they had not now time to cleanse themselves according to the law:
therefore the Levites had the charge of the killing of the passovers
for everyone that was not clean, to sanctify them unto the Lord;
this they did for the masters of families, who were ceremonially unclean, who otherwise might have killed their passover lambs themselves, see ( Exodus 12:6 ) , but now the Levites did it for them, that their passovers might be sanctified and consecrated to the Lord; for, as Philo the Jew says F18, one day in a year the whole sacrificed, everyone acted as a priest, and brought and slew his own sacrifice, meaning at the passover.
F18 De Vita Mosis, l. 3. p. 686. & de Decalogo, p. 766.