Complex sentence: Dexter goes to another Kyle Butler and renders him unconscious via injection, in order to catch the Trinity Killer, but as soon as Mitchell comes through the door, he realizes that it is not Dexter and leaves before Dexter can grab him.
Split into simpler sentences: Dexter goes to another Kyle Butler and renders him unconscious via injection, in order to catch the Trinity Killer. But, as soon as Mitchell comes through the door, he realizes that it is not Dexter and leaves before Dexter can grab him.