LONDON — A radical Muslim killed a soldier outside Canada’s Parliament. A right-wing extremist opened fire on buildings in Texas’ capital and tried to burn down the Mexican Consulate. An al-Qaida-inspired assailant hacked an off-duty soldier to death in London. Police said all three were terrorists and motivated by ideology. Authorities and family members said they may have been mentally ill. A growing body of research suggests they might well have been both. New studies have challenged several …