[Galadriel55]

I agree with Pitch, but I want to add that maybe Beren was in kind of a stupor from all of the things that just happened to him, and from feeling Morgoth's power and majesty. He must have been overwhemed, and the only thing in his ind would be to get Silmarils. If Beren would have had some time to recover, aybe he would kill Morgoth.
Also, it's easy to say "kill Morgoth", but its hard to make yourself do it. It's not only killing a superior, as Pitch said, but doing in the mightiest, ost powerful being. Wouldn't that fill you with, well, awe?
So there are two possible versions - that Beren willingly let Morgoth live, due to ethics, honour, mercy, or whatever other reason, and that he just didn't consider doing it for reasons mentioned above. It could be both - at first he was too shocked, and when he recovered his wits he decided to spare him...