They killed Stephen because he preached against the need for the temple. In the context of the Second Temple Judaism, in which the entire religion was focused on the temple, Stephen both preached for and against the temple, in essence, that the temple is a shadow of the reality that is to come, and that the reality has come in Jesus. Even though it is only a shadow, the temple has a lot to teach us. First, it shows us the fundamental problem of human beings--that we are separated from God resulting in a thirst that we can’t quench on our own, and resulting in brokenness in our relationships and in the world. Second, it shows us that we need someone to atone for our sins. We can’t just enter into God’s presence without atonement. We can’t fix ourselves enough to be acceptable to God. We can’t just fix the world on our own. Third, God has sent Jesus to be an atoning sacrifice for us, resulting in reconciliation and pouring out of his presence. So, since the completed work of Jesus, the temple is no longer necessary. We no longer need to rely on the shadow of the reality; we can rely on the reality of the atoning work of Jesus Christ.