Complex sentence: His ransom demand rebuffed, Esen considered the emperor more valuable alive than dead even without the possibility of ransom, or because the acting emperor was content not to obtain his brother's freedom, Esen began laying siege to Beijing.
Split into simpler sentences: His ransom demand rebuffed, Esen still considered the emperor more valuable alive than dead. Probably because the acting emperor was content not to obtain his brother's freedom, Esen began laying siege to Beijing.