Humans including you and me have been killing animals, objects and other humans for thousands of years, yet if we take a look at society and movies (as an example), it seems like we were never familiar with killing or corpses...but then again its inside our DNA.

Chapter 1 - What justifies a murder and what do these people think ?

Generally this is a tricky question because feelings & thoughts vary from losing weight from your shoulders to total depression and more.
But what's more important is the situation, the person is in.
Many murder happen during / through drug dealing / dealers.
They mostly are already in a critical state, not only because they mostly likely are drug addicts but they have to go through a horrible time including keeping reputation up, dealing and not getting murdered and to stay away from justice because of peer pressure.

These people will most likely not regret the bullet that they shot because they justify the action with their life situation whereas people with higher living standards regret a murder way more either through a car accident or axidental murder or anything else like that.
To summarize this chapter:
The worse your situation is, the more likely you'll justify any act including murder.

We have been hunters, killers for thousands of years

I ask myself, why is it that people would cry and feel totally guilty if we have been killing things for thousands of years?
Isn't it the same but a little bit later, what separates our killing thousands of years ago and killing someone today?
Think about it and ask yourself the same question, aren't we today still like our ancestors?

http://i.dailymail.co.uk/i/pix/2016/08/10/09/3710E23E00000578-3732832-image-m-26_1470818220648.jpg

Chapter 2 - Movies are a great example!

Movies are such a great example because the character goes through a rough time.
The character wakes up in the middle of the night, gets nightmares and can't focus on something in a crowded / friend zone. I have 2 great TV-Shows that represents what I mean :

Mr.Robot

Breaking Bad

These things happen because the brain can't get rid of the idea of murdering and justifying it.
We just happen to have evolved into a structure where murdering is not needed to survive
anymore.
What's worse is that the brain think ahead, it knows the consequences if other people, friends
know about the murder so it must keep everything inside your brain.

http://www.dreampositive.info/wp-content/uploads/2013/03/phoenix-by-deviantart.com_.jpg

Your brain knows its was wrong. But here's something that I didn't know about until I started to learn lucid dreaming: Your brain can not.
If you try not to think about something that you hate or will never do in your life, for example smoking, you will start to think about smoking inside your head.
You try now not to think about smoking but the more you try, the more it stays.
That's why your brain can not not think about something.

Chapter 3 - Why do people murder at all?

The reason(s) is (are) simple:
1.Pressure
2.Mental-difficulties
3.DNA-Internal-Kernel-Error (my idea)

1.Pressure

Either Peer pressure, self esteem or just pressure of someone else one thing is common, they all end up with one person trying to prove something.
Its the point where someone has to do something in order to prove their abilities.
In a group the members want to know whether a person should be part of the group so they let this new person do something scary.
This ends up with the person going through a rough time and maybe even without reaching the goal!
https://i.ytimg.com/vi/zU43kOKUbMA/maxresdefault.jpg
Proving something to yourself is important, not only will your brain accept this new habit
that you proved to yourself because you showed that you can indeed do that specific scary thing. This can be anything from sports like freerunning (:P) or climbing to doing a speech free without any notes to unpredictably murder or torture what ever your brain prefers.
To be honest I don't really have much to say when it comes to proving something to yourself because freerunning was the only the part where I would apply.
http://i4.mirror.co.uk/incoming/article5671111.ece/ALTERNATES/s615b/IMG_3427-copy.jpg okay maybe not as hardcore as this guy...

2. Mental-difficulties

Most commonly known and yet a big problem are people with mental problem.
These people need help but in times where there is no help these people collapse and seek for justice, maybe attention, proving that they are normal and so on.
These people tend to murder more than all other reasons because they can't do anything against it.
They can't get control over their illness and start to shift towards this new habit of killing and in their perspective freedom. https://static.independent.co.uk/s3fs-public/thumbnails/image/2015/08/08/19/Mental-Health-Alamy.jpg The science is in and there are many statistics and articles proving the violence of people with mental-illness and some are proving the opposite.
This statistics shows some sweet graphs and numbers and nice colors and pictures
...Just look at the colors.: https://steemitimages.com/DQmamvVP559pgfDrgKK96EKmPJiTdFwGx233rvVRLJoiGce/image.png This is a problem that should be solved faster than anything else because these people need help!

3.DNA-Internal-Kernel-Error (my idea)

I came to the conclusion that maybe just maybe we are programmed to kill because we did it for thousands of years. Humans have been hunters and gathers for more than 90% of their existence,
1879 was the year of the invention of the light bulb yet 30 years ago we had super-apocalyptic weapons ready to launch because we didn't agree with other countries. After thousands of years we shifted towards a peaceful world and our complex mind may have shifted but not our DNA.
This is my idea and I think it can't be completely wrong because I doubt that we are so different from our ancestors and their hobbies.

Police officers kill without problems?

One thing that I have noticed is that police officers are the people who kill without problems.
Not because they are robots or have no feelings, because they know that everybody around them know its the right thing to do (most cases...).
http://losyziemi.pl/wp-content/uploads/2014/08/USA-W-zwi%C4%85zku-z-zamieszkami-gubernator-wyda%C5%82-polecenie-wprowadzenia-na-ulice-Ferguson-Gwardii-Narodowej-3.jpg An officer will not cry about someone that he killed, and that's the key point here, they kill criminals because they know its justice and correct whereas private murdering ends up in strong depression and mental-problems for a few weeks or maybe months.
This is what makes me wanna research further into this topic, the philosophical and scientific question exactly why police officers feel normal and maybe good after they did their job where on the other side the normal murder ends up so badly.
I'm done, I would have written more but I'm tired and my fingers hurt so would you please upvote this post and follow me for philosophical and scientific posts ?