While there are examples from every discipline, profession, and religion, here is one that I remember:
Nobel Prize winner Daniel Carleton Gajdusek, a pediatrician and researcher, was jailed in 1997 for molesting a 16-year-old boy, one of 56 Micronesian children he molested. Gajdusek was the chief of the Laboratory for Central Nervous System Studies at the National Institute of Neurological Disorders and Stroke in Bethesda, Maryland. Washington Post, Under Plea Deal, Former NIH Scientist Will Spend Up to a Year in Jail, February 19, 1997 p. A1.
Aikido is no different than any other human activity in this respect: years of study will not turn evil into good.