One of my headcanons is that Keith knows he is half Galra, half Altean.

I like to imagine that Zarkon sends him to earth to observe the human race and see if any of their technology or soldiers are of any use to the Galra Empire. 

So Keith enrolls at the Galaxy Garrison and quickly makes it to the top of his class. However, Takashi Shirogane is still the best pilot in the whole institution if not on the entire planet. Keith, hurt in his pride but also intrigued that there is actually someone better at piloting than him, befriends Shiro. Eventually, he realizes there is more to the officer than just his piloting skills.

After Shiro takes off to Kerberos, Keith contacts the empire to capture Shiro and put him in his personal squad. They do as ordered but when Keith returns, finds out they cut off Shiro’s arm in battle and traumatized him for life. 

Keith feels betrayed and swears revenge on Galra. After that, he and Shiro decide to become double agents. They join team Voltron to fight the Galra. In the end, they pretend to betray team Voltron and ally with the Galra again to destroy them from within.