Complex sentence: When interpreting these old texts, it is therefore important to have in mind, that the authors were sometimes enemies or victims of the Vikings more aggressive side, and that have surely given many of these texts a certain bias.
Split into simpler sentences: When interpreting these old texts, it is therefore important to have in mind, that the authors were sometimes enemies or victims of the Vikings, having experienced their more aggressive side. This have surely given many of these texts a certain bias.