A pickup truck veered across the center line of a highway last week and killed seven motorcyclists, injuring three others. The slain bikers, five men and two women ages 42 to 62, were members of Jarheads MC, a New England motorcycle club for Marine Corps veterans and close friends. They collided with the Dodge truck, which was towing a trailer. Neurologist Beatrice Engstrand, who was staying near the crash site, responded to a scene resembling “a war zone,” with 20-foot flames, wrecked vehicles, and people desperately administering aid, including to one man missing part of his skull. The truck driver, Volodymyr Zhukovskyy, 23, pleaded not guilty to seven counts of negligent homicide. Employed by a transportation company in West Springfield, Mass., Zhukovskyy had his license suspended after a 2013 arrest, and was arrested again on drunk driving charges just last month.