Complex sentence: Joker is still haunted by the memory of his friend Cowboy, whom he killed in order to keep his squad from being cut down by a sniper; as a result, his behavior has become increasingly erratic and violent.
Split into simpler sentences: Joker is still haunted by the memory of his friend Cowboy, whom he killed in order to keep his squad from being cut down by a sniper. As a result, his behavior has become increasingly erratic and violent.