Like I said, he was in a Crusade of sorts against Orphenochs.
All this, to him, was justified. His father was an Orphenoch, he's in a crusade against Orphenochs, put two and two together. He probably felt Takumi was impeding on his plan/progress, too, and wanted him out of the picture. Justified in his mind. Kiba vs. Takumi, two birds with one stone. I'm not saying I'd do all this, genius, I'm saying that it doesn't surprise me that Kusaka did it. I'd do it if I were him too.