Complex sentence: It was at this time that Pichushkin began to develop a more sinister hobby that at the time remained unknown to anyone: whenever Pichushkin knew he was going to come into contact with children, he would take a video camera along and proceed to threaten them.
Split into simpler sentences: It was at this time that Pichushkin began to develop a more sinister hobby that at the time remained unknown to anyone. Whenever Pichushkin knew he was going to come into contact with children, he would take a video camera along and would proceed to threaten them.