Vikings were originally a Germanic people who did wear horned helmets in the Bronze Age, most likely for ceremonies related to their religion, but they gave them up later. As I said, they were originally a blonde, blue-eyed Germanic people who spoke a Germanic language and exhibited Germanic characteristics. As they spread across Europe and North Africa, particularly Europe, they incorporated other peoples by kidnapping locals to serve as wives or slaves and introduced those genes into their genome. However, they were initially a blonde, blue-eyed people.Neither horns nor language have anything to do with it.
The Vikings have a reputation for being fierce, but I think this has more to do with the climate in which they lived. Scandinavia, before the scientific age, was not a very conducive place for agriculture, and crops often failed. When this happened, some Vikings would take to their boats to search for food for their starving families. Also, the term "Viking" does not refer to a group of people but rather an activity. Vikings were raiders, and not all members of the Viking community were raiders.