What is a possible reason that Beowulf allowed one of his men to die at Grendel's hands before he attacked Grendel himself?

he wanted Grendel to let his guard down, and he also wanted to make the feat of deafeating Grendel look tougher than it was.

You have to understand an epic is a poem that tells the story of a hero who is supposed to be larger than life and these characters normally have large egos, and they love to boast about themselves, which is what Beowulf does.

So, if he lets a few of his men get eaten it makes it seem as if he was the only one amongst them who could deafet this horrible creature, and it adds more prestige to his name.