Explain why Ambrosius acted so brutally in attacking Vortigern at Doward.

From the text:

It seemed, from what the messenger had told me, that Ambrosius had been right in his decision to make an end of Vortigern before turning on the Saxons. His reduction of Doward, and the savagery with which he did it, had their effect. Those of the invading Saxons who had ventured furthest inland began to withdraw northwards towards the wild debatable lands which had always provided a beachhead for invasion.

From the text:

It seemed, from what the messenger had told me, that Ambrosius had been right in his decision to make an end of Vortigern before turning on the Saxons. His reduction of Doward, and the savagery with which he did it, had their effect. Those of the invading Saxons who had ventured furthest inland began to withdraw northwards towards the wild debatable lands which had always provided a beachhead for invasion.