After finally killing off all the Greek gods, Kratos went off somewhere else for a while (maybe looking to other dimensions for new challenges) just long enough for belief in him to fade. When he finally returned to this plane of existence, lack of belief had forced his power level so low he had to take on a lesser form when he incarnated on earth (much like Om in Small Gods). Even as a scorpion though, he kept some fundamental points of his personality, namely his love of REALLY violent finishing moves and his Kill Em All disposition. Him going after Struggs was an attempt to regain his noteriety, and consequently, a better form.