Tsukiko's death was probably THE most narratively satisfying outcome possible. A sensible consequence of everyone's motives, a tragic aspect added to T's naivete, a major plot point tied into her demise, and setting up the tragic villain of the strip for his own major fall. As much as you want to have seen more of Tsukiko (and what more could she have contributed to the story, exactly?), that doesn't make the Giant's decision a "big-ass mistake."