Fable III

I would like to note that if you chose the good/moral quest line throughout the game then many more people will actually already love you. I went to dwellers camp after becoming king and found many potential wives waiting for me to just marry them! I didn't even have to do the relationship quests, just marry, kill (take them to a secluded area or people will become neutral/hate you), and repeat.

EDIT: For the achievement you are required to marry 6 people and kill 2 of them after you become king.

This achievement requires you to have married at least six people after becoming king/queen, and asks you to kill two of them.

In order to marry a villager, you must have a wedding ring in your possession, and interact with the villager (taking note of the relationship meter at the bottom of the screen) until the option Propose Marriage (RB) becomes available. This may entail relationship quests, unless..

As p00rman30 rightly stated, if you played nice and picked all the positive decisions throughout your rise to power, albion will already love you, and the entire process of gaining your victims affection becomes shockingly easy.

I'd like to note that the gender of your spouses (in relation to yours) doesn't seem to matter. Or, at least, it doesn't matter too much. I was female, and married multiple women (it's a video game, why not?) I killed a few of them, as you do.

Much later, I read that you must wed villagers of the other sex, so I fired up my Fable III, checked my stats - four current spouses, perfect! - I wed two more men, and just as i levelled my rifle at the second victim/husband, the achievement popped.

In laymans terms, you must marry at least six people over the course of the game, at least two of them must be killed by your hand, and at least two of them must be the opposite sex. Although the ones you kill, do not have to be the ones of the opposite sex.

Good luck!

After you become ruler, you need to get married 6 times. You do not have to be married to all 6 people at the same time, and you do not have to kill either of them in any order.

So, you can kill your first two spouses and divorce the next 4 if you do not want them to hang around.

After the 6th marriage, or the 6th spouse is the 2nd one to die, you get the achievement.

NOTE: These 6 marriages have to be after you are ruler. If you get married before you are a ruler it will not count towards the achievement if you are missing spouses after you are ruler. So you must marry 6 times after you are ruler.

After you become king you have to marry 6 people. This is done by buying 6 wedding rings (i got mine from bowerstone market).

After you have done this go find a woman (not sure if you can do it if you're a female). Do nice expressions until she gives you a relationship quest. Do this then go back to her. Keep doing nice expressions until she want's to go on a date with you. Perform the date and after that you can press RB to propose marriage. Select and house and location and then get married!

Rinse and repeat 6 times (I got married in 6 different locations). Next go back to 2 of your wives and pull out a weapon, press down on the d-pad and kill them!.

Do this to another wife and then it should unlock.

This is easiest done together with "Popularity Contest" as after you win that achievement you can start to interact with your friends to trigger their 'date' quests.
A lot of confusion seems to span this quest as a lot of people don't realize you MUST be king or Queen first before you marry.
Also, you do not have to be married to those six people at a single time. You can easily just marry and kill off as many people as you want. As long as you married 6 people and killed at least two after you became King/Queen it will count.