See this is why he should have fought Kengo, if he fought Kengo the other two wouldn't feel the need to intervine and turn Wataru...
Wataru could have prevented a lot of this stuff from the get go if he was honest, yeah it don't make for good drama, but Kengo was a good friend too him, and he kind of got walked all over in return. So I can sympathize with Kengo.
If they are gonna kill Kengo off (Which I don't think they will) it should be while doing something heroic, any other way would just be random and pointless.