Dexter had entered into a dangerous game of cat & mouse with Trinity. For 3 series he had got off without much consequence for his actions. No way S4 was going to end without something major going down. Dexter didn't function emotionally like a normal person, but Rita dying did impact him. The guy was in genuine shock when S5 kicked off and he was off-balance for most of it. Now, this was likely because he probably felt he had got one over on Trinity and was riding high on taking out a killer who was in the game 30 years. But I think Dexter had grown to care for Rita.