Barristan Selmy is known for his character and honor in Game of Thrones. He was a member of the Kingsguard for the Mad King Aerys II and sworn to protect him and his family. He also seemed to be very fond of prince Rhaegar Targaryen. Then Robert kills the prince and Jaime Lannister (another Kingsguard) kills King Aerys.
As aftermath of Robert's Rebellion, Robert usurps the Iron Throne. And he pardons many lords and counsil members like Varys, Pycelle and Barristan Selemy. Varys and Pycelle are men of opportunity, however Barriston is a different case. Why didn't he try to avenge the King or take the black instead of serving the usurper who killed his king along with the Kingslayer again as sworn brother?
What is the motivation of Selmy for doing so?