I think Mahanta and Katri at least have had some good points (I don't even know what I should comment on the rest so I won't) Mahanta had a good example of siblings, where even with the same background and surroundings and such, they are still different. So yes we do have some fundamentally same things like survival instinct etc but there are so many other things affecting what we're like that it isn't that simple and doesn't mean we're the same. Then Martin's latest comment was relevant in terms of having restrictions when studying the topic. By recognizing these differences and restrictions and taking them into consideration, some studies can be made, even if they don't apply for everything and everyone. So knowing that there are so many variables affecting each person, the studies and claims made will be generalizations or estimates.

As many have already poinetd out all people are different, and that makes it difficult for human scientist to get an exact and reliable reuslt. Of course it can alwaus be generalised, but generalisation can give a wrong picture. I agree with Katri that many different thinks affect a people`s behavoiour, e.g. as Ella mentioned cultural differences, the environment, our emotions, how we were raised, etc.So yes it is very difficult to test human behavoiur, due to so many things.

When studying any part of human science there are too many unknown variables that exist in order to make completely accurate hypothesis about human behavior. For example, psychology may provide excellent explanations for behavioral phenomena. BUT until somebody actually manages to map the synaptic complex of the human brain, I believe there exists simply too many variables for scientists (psychologists) to make accurate predictions. Factors that affect the variables include (as Ella mentioned) the environment of growth, culture, religion, and the list could go on for pages.

Psychology still provide excellent predictions and theories on human nature and should not be neglected, but is not 100% reliable.

Actually, psychology isn't that reliable after all. If we use an example of criminal psychologists, these people face lies all the time, some lies even that good that it is impossible to tell the lies apart from the truth, so this is a big failure in determining the human nature.

Another problem that psychologists may face, is pathological liars. Pathological liars are people who lie, just because they do, they may not want to, but they do it. These people can develop a progression of lies that together sum up to what they believe to be reality; they actually start believing in their lies, and in the end they are so convinced that it is true that they deny everything that might sound even slightly made-up during a psychological evaluation. So this also is an inaccuracy in psychology.

Then there are people with mental disorders and very different natures from time to time. E.g. people with a syndrome called Munchausen by Proxy syndrome, are people who hurt their young children or sibling by violence or through harmful medicines or drugs, in order to get attention in the hospitals. There was one case of this syndrome where a mother in Britain had hurt her child in many different ways over the course of 4 years, and during this time they together visited the hospital ca 600 times.
This syndrome is quite a failure in determining a general human nature, since the general thought would be that mothers protect their children rather than hurts them.

I find it generally misfitting to use murder as a way of representing human nature and humanity in a discussion about human behaviour.
The willful taking of life represents the ultimate disconnect from humanity. It leaves you an outsider forever looking in.
There are an abundance of killers on our globe and they all have their own deranged reasoning for why they killed sombody. But behaviourally they are so vastly different that it is impossible to isolate one behavioural tenent that would cause anybody to kill another human being. Infact the only thing all the killers have in common is that look in their eyes.