A run can go south at any time. It happens, but it never happened to Anders before the night Daze-E didn't come home. Now he wants revenge on the runners who let her die on the job, regardless of what that particular course of action will cost him.
The Temple of Sacred Ashes has been destroyed, the Veil has been breached, and Thedas stands on the edge of chaos. A lone elf emerges from the destruction wielding the "mark", making her only person in all the land that can set things right. Andraste probably should have chosen better.