Why does Eric try to execute Tris?

In Chapter Sixteen, Eric announces that his orders are to take only two Divergent back to Erudite headquarters for testing, and execute the rest. He begins to execute the younger people, claiming their brains are not yet mature so their Divergence is not fully developed. He says the same of Tris, but as he speaks to her, she lurches forward and stabs her knife into his heart.