Why did the bleeding Captain describe the valour of Macbeth?
After the battle, Malcolm recognizes the Captain (or Sergeant) as the one who saved him from being taken prisoner. Malcolm commends the Captain and asks the Captain to inform himself and Duncan about what happened during the battle. The Captain explains that the odds were against them. Macdonwald had a large army:
The Captain then describes Macbeth's valour and how he (Macbeth) fought his way through the army and claimed Macdonwald's head:
The Captain described Macbeth's valour because it was Macbeth who was largely responsible for the victory. This ultimately leads to Duncan giving Macbeth the new title, Thane of Cawdor.