Is it permissible for a worshiper to interrupt his prayer to kill a snake or scorpion he sees in front of him, or should he complete his prayer?

Yes, it is permissible to interrupt the prayer to kill a snake or a scorpion, based on the words of the Prophet, peace and blessings be upon him: "Kill the two black ones in prayer: the snake and the scorpion." If it is possible to kill them with little action, then the prayer remains valid.