"Lamb of God" and "saved by the blood of the Lamb" are two of many traditional Christian metaphors, rooted in biblical imagery, which many contemporary Christians find troubling, perhaps even disturbing.
The difficulty emanates from the orthodox Christian teaching that Jesus had to die in order for God to forgive human sin. Theological conceptions of this use ideas including atoning sacrifice, expiation, and propitiation to explain this concept. Although the various explanations have significant, sometimes mutually exclusive ideas, all of the explanations presume that God can forgive human sin only in conjunction with a blood sacrifice. Since the sacrifice must be without blemish (i.e., without sin), and only God is perfect, only God (or God's son) can satisfy the requirement. This explanation justifies, even necessitates, Jesus' death on the cross.
Thankfully, contemporary Christian theologians increasingly find such ideas very troubling. Those traditional concepts depict God as either a masochist (choosing to suffer) or abusive parent (inflicting the suffering on God's son). Both options are spiritually and morally repugnant.