unadvised > ill-considered waxed > grown, become wood > furious, mad [When through ill-considered rashness he had become furious]
4 For of his hands he had no government,
government > control
5 Nor cared for blood in his avengement:
cared for > cared about, shrank from [shedding] avengement > vengeance
6 But, when the furious fit was overpassed,
overpassed > over, at an end
7 His cruel facts he often would repent;
facts > deeds
8 Yet, wilful man, he never would forecast