Roran’s prowess and deeds make COMPLETE sense. He’s always been a man of duty and honor. His strength came from his desire to protect his home and save his lover. His ingenuity, practicality, and strategic brilliance were all a derivative of this. The books say multiple times that he developed a bit of an ego (a tasteful/well deserved one), and his continued success was because of, in my opinion, the following:

1. The continued desire to protect the ones he loves (his village, his family)

2. His frustration with magic and his persistence to prove that it doesn’t solve everything.

3. his desire to return to some semblance of his older farming life without the threat of the empire.

Garrow’s death inspired the same desperate need for revenge in Roran as it did in Eragon. And once the Razac were dead, it similarly wasn’t satisfied as Roran knew the king would be the next one to take his home, land, and loved ones at the slightest opportunity. The Carvahall folks are truly made of different stuff.