Complex sentence: The two slain justices had a history of environmentalism--their only common view--and thus Darby surmised that Mattiece hoped to turn the case in his favor by eliminating the two justices.
Split into simpler sentences: The two slain justices have a history of environmentalism; their only common view. Intelligently, Darby surmised Mattiece hoped to turn the case in his favor by eliminating the two Justices.