Complex sentence: Finding out that Kazuya would be participating in it, Leo follows suit, believing this would be his only chance to avenge his mother, unaware that she was one of the key suspects that betrayed and tried to kill Kazuya in the first place.
Split into simpler sentences: Finding out that Kazuya would be participating in it, Leo follows suit. Leo is unaware that she was one of the key suspects that betrayed and tried to kill Kazuya in the first place.