Let’s take a stroll back to beginning composition class to figure out how to illustrate a convincing change in character motivation.
In once scene Dick makes a point. In another scene Ted makes his counter point. Each encounter they have is an attempt to sway or force each other to adopt the opposite way of thinking. A different point is driven home each time.
Ted's scenes illustrate why he wants humankind destroyed. Ted may think Dick is a hopeless dreamer. Dick may drive home a few points that make Ted reconsider.
Dick's scenes address the reasons humanity should be saved. A few scenes could show him questioning his stance. Yet, it is Dick’s belief in the innate goodness of mankind that eventually gives him the tools or the access to stop Ted’s nefarious plan.
There will be friends that fight alongside Dick to save mankind. There may be a secondary character that is on the fence. Perhaps he or she encounters Sally and wonders if Dick is fatally naive. Jane could have a perilous dilemma of her own and her self-sacrifice illustrates Dick’s point that humanity is worth saving.