Skjor should stay dead during the Companions quest right?

After I returned to Vilkas after completing a quest, I look around and Skjor is sitting next to Aela. I have completed the Companions quest. Any possible answers?

You are correct. Skjor should stay dead after finishing the Companions quest, The Silver Hand. However, according to the UESP wiki, there are bugs associated with Skjor:

Bugs

• Sometimes Skjor will reappear in Jorrvaskr, alive or dead, after he dies during the events of The Silver Hand. He may also reappear in Jorrvaskr and still give out quests. You will not get a bounty for killing him if he is alive.

• Sometimes Skjor can be found wandering Skyrim while half-buried in the ground after finishing the quest The Silver Hand.

The same UESP wiki article also states that both bugs are fixed by the Unofficial Skyrim Patch.