You seem to be under the mistaken impression that the remnant of Israel can only mean those that are faithful and righteous among the children of Israel.
As we can see in the verse below, GOD is angry with the remnant for their iniquity and transgressions, but will have mercy on them all.
Remnant can also mean any and all of Israel that are left (ie. ALL that remain alive at the time).
Micah 7:18 KJV
(18) Who is a God like unto thee, that pardoneth iniquity, and passeth by the transgression of the remnant of his heritage? he retaineth not his anger for ever, because he delighteth in mercy.
Even in the NT, the remnant does not always mean the faithful and righteous ones.
Matthew 22:6 KJV
(6) And the remnant took his servants, and entreated them spitefully, and slew them.