Complex sentence: He was also intelligent but exhibited antisocial and psychopathic behavior such as cruelty to animals: at the age of 10, he buried a pet cat alive; once it died, he dug it up, decapitated it and mounted its head on a spike.
Split into simpler sentences: He was also intelligent, but exhibited antisocial and psychopathic behavior such as cruelty to animals. At the age of 10, he buried a pet cat alive; once it died, he dug it up, decapitated it and mounted its head on a spike.