Beowulf immediately comes to the aid of the Danes when he hears of Grendel's attacks on Herot. I think what is interesting is that, while he comes to help and he's traveled across the seas to do so, it's really all about him. In greeting King Hrothgar, he "boasts" about being "blood-flecked" in battle directly before being notified of the Danish plight. "Themselves had seen me from slaughter come/ blood-flecked from foes, where five I bound,/ and that wild brood worsted." He talks about his bravery and then says the following:

Grendel now,
monster cruel, be mine to quell
in single battle! So, from thee,
thou sovran of the Shining-Danes,
Scyldings'-bulwark, a boon I seek, --
and, Friend-of-the-folk, refuse it not,
O Warriors'-shield, now I've wandered far, --
that I alone with my liegemen here,
this hardy band, may Heorot purge!

He's come to help, but you can tell by his tone and by his word choice that he is here for himself more than anything or anyone else. He says Grendel is "mine to quell in single battle!" He's brought his band of men to take on the task, but he really only needed them to guide the boat to get there. The battle, he says, is all his.