Why were europeans so frightened of the vikings?

Because they raided in all of their continents and settled there, and also destroyed all of their belongings.

Hope this helps!

The Vikings were fierce warriors who constantly raided the shores of Frankia (France and Germany), the British Isles, Ireland, and even nowadays Russia.

They would burn and loot villages and take the materials, that they would then trade off elsewhere. They also hired troops to build armies and some even took european cities.

I believe the Frankian's economy was so damaged by the constant looting that they had to start building walls to protect their settlements