Why does Beowulf allow Grendel to slaughter one of the Geats before taking action himself?
It's not clear why Beowulf allows this to happen, assuming he is able to prevent it. It does say, in the Heaney translation, lines 736-738 that "Mighty and canny, / Hygelac's kinsman [i.e., Beowulf] was keenly watching / for the first move the monster would make," and that he watched as Grendel "mauled a man on his bench" and "bolted down his blood." So on the face of it, Beowulf seems to sacrifice one of his men so that he can take the measure of Grendel.
Nevertheless, there is something otherworldly about Grendel's attack. The Geats expect Grendel to come, yet somehow they are all asleep except for Beowulf; when Grendel comes to the hall, he rips the door off its hinges ("he ripped open the mouth of the building") and somehow is able to "pace the length of the patterned floor" seeing how many men were there, all somehow without raising an alarm. Grendel is a magical creature -- he has somehow "conjured the harm from the cutting edge of every weapon" -- so it is also possible that his attack on the Geat was simply too fast for Beowulf to intervene.
In another sense, the question you raise speaks to the code of honor and glory that underlies the poem. That is, it doesn't matter, really, that the Geat is killed; his death is honorable, and besides that the point is that Beowulf defeats Grendel and brings glory to all the Geats.
Your question is worded in such a way that it assumes Beowulf was somehow okay with one of his men being sacrificed to Grendel's perverse penchant for killing innocent people. That just doesn't ring true: Beowulf doesn't allow one of his men to be slaughtered by Grendel. It just happens. This does seem to be the one misstep Beowulf makes in an otherwise amazingly successful campaign against terror. Of course he would rather his friend hadn't died; however, Beowulf certainly got the appropriate revenge after his friend's death.