Can you fill me in on Dexter? SPOILER ALERT!

Well, my dad has just finished watching the season finale of Dexter and recently informed me something about the Trinity killer murdering Rita? Is that true?
Please tell me what happened exactly and do you believe he actually did? Or was it all just in Dexter’s head? My dad has also told me that his baby son was just sitting there in a pool of blood (which sounds very similar to Dexter’s child hood). Please tell me because my father has just finished watching it…it is coming on again but won’t let me watch it because he doesn’t feel like seeing it again >.< haha thank you guys!