That stone tile was lying amidst the rubble, and in order to pull it out, Han Sen would have to remove the cluster of rocks that buried it. Unfortunately, the two Dukes were still present. It'd be impossible to retrieve the stone without disturbing them.
Han Sen tried to think of a way that he could grab the stone tile without drawing their attention.
Killing the two Dukes wouldn't be difficult for Han Sen, but if he wasted them, it'd alert Edward that someone had found the ruins. It would prompt the Inspector to investigate, at the very least.
After all, the only non-xenogeneic residents of that planet were the Ice Blue Knights. Edward's target would be someone from the base.
Han Sen's best bet was to steal the stone tile while avoiding drawing the guards' notice. It would save him a lot of trouble.
