How is Dark Messiah connected to Heroes of Might and Magic?

Did you know that he was actually defending himself? In one of the videos, when Railag confesses his love for Isabel, he says that he feels very sorry because Nicholas attacked him when he was withdrawing troops from the Empire.

He said that, but in the videos, we first see Agrail with a huge army attacking Isabel, and then a demon sending warriors one after another against the exhausted Cole. He could have weakened Nicholas and escaped, but instead, he finishes him off. He could have easily left Nicholas alive if he wanted to, but he didn't. Moreover, Agrail could have sabotaged the demon attack, in which case the King would have lived, the Empire would have saved its army, and the demon's vanguard would have been defeated.