Despite most of his kin having left the Alliance,  Ala'theas chose to continue to serve in its millitary, as a marksman. Even so.. when his homeland of Quel'thalas was destroyed by Arthas's scourge, he felt guilty for being unable to help. And such, continued to hunt the undead in the area.  When the scourge was undone, he found himself purpose-less, and started mercenary work.