What happens if you kill Alex in the Till Death Do Us Part mission in Fable 2?

Answer 1

You would not be able to complete the quest if you kill Alex.

There are a couple ways of completing the quest. First, do what the ghost of her jilted lover asked and swoon her and then give her the heart break note. She will be crushed and your alignment will take a dip towards evil.

or

Swoon her, marry her (or him depending on your character), go back to her ex. All will be happy including her ghosty ex. Your alignment will bump up to good a bit.

There is an extra little trick to this quest. If you take her to the clock tower and use the seduce expression, she will fall in love immediately. Do not get engaged to her yet. Use the follow expression and have her follow you to the Oakfield demon door. He wants to see an engagement. Once the door does his little speech, give her the ring. He will open up for you. Go inside and there will be a nice home. Use the real estate tag outside the door and this will set the house as a marital home and you get to keep it. The area will be called Serenity Farm. While still in Serenity Farm, go inside the windmill and the legendary weapon "Hammerthyst" will be in the treasure chest on the right hand side behind the big gear. Go back to her ex and the quest will be completed. You can still open Serenity Farm using people besides Alex. This is just an easy way to knock out two birds with one stone.