My theory so far is that just like how Guts's sword gained properties of darkness for killing all those dark spirits. That same darkness has been slowly creeping into his own spirit. Which is why his inner wrath is so strong.
It does take a strong will to control that beast indeed.
The pose on the last page, Guts chilling like that... it brought back memories from when he joined the Band of the Hawks and talked to Judeau... damn! ;-;
