I kinda think, that Blue is sus didn't actually killed D.
On 17th page he looks like he is hesitating. And then his "attack".
Like, remember 3rd chapter, when Red used his weapon on that fat footsoldier? That whole scene with 'lightning dragons' (or whatever it was), screaming it's attack, etc.
And now we have Blue, who just stabbed him once, before D blew up.
Not to mention, that D could still have dynamite from last chapter, which explains the explosion.