There was in interesting morality experiment recently on the topic of what we consider justifiable. Basically, they posed a classic moral dilemma to a group of people, while performing a fairly detailed monitoring of their brain activity.
The dilemma is two-part. In the first part, you're standing at the track switch control of a rail yard, and there's a high-speed train coming through. There is a track crew working on the track ahead of you. You know that the train cannot possibly stop in time, and that the crew cannot see or hear you, so you have only one available choice. If you leave the train on its current track, four members of the crew will be killed. If, however, you switch the train to an alternate track, only one crew member will die.
Now, when posed with this question, most people make the choice to kill the one crew rather than let the four die. Brain activity showed a process of deliberation, a weighing of relative values, before a decision was reached.
The second part is similar, only this time you are standing on a bridge above the switch control, with another man. The train is coming, the four members of the road crew will die, but there nobody on the alternate track. The only way to activate the switch in time, however, is to push the man you're standing on the bridge with off, so that he strikes the switch control and diverts the train.
Almost everyone, when asked this question, said no. Immediately. The brain showed an overwhelming response from a completely different part of the brain, effectively shouting down the parts involved in deliberation. It was an arguably instinctive refusal to actively kill another human being.
Obviously, the interesting part of this is that in both outcomes, someone dies as a result of your action, whether it be by the indirect murder of throwing the switch, or the direct murder of pushing a man off a bridge.
Suffice to say that there is at least some evidence for an instinctive basis of morality, at least as far as some things go.
Anyhoo.
On a related note, pretty much any predator will occasionally kill for the fun of it, assuming that the act of killing poses no particular risk to themselves.