He was a tribal ruler that did not implement Islam.
The tafsir of the Aya of "Whoever does not rule with what Allah has revealed..." is as follows:
1- If they implement Kufr laws, but believe that Islam is superior and they do *not* oppress their people, then they are Fasiqoon.
2- If they implement Kufr laws, but believe that Islam is superior and they *do* oppress their people, then they are Thaalimoon.
3- If they implement Kufr laws and believe that these Kufr laws are superior to Islam, then they are Kafiroon.
So, we do not know Faisal's intentions, therefore he was a Fasiq at minimum. That is as far as we go.