What do you think is the single best explanation for homicide?
Looking for a "single best" explanation for homicide is problematic. There are many different motives for homicide, perhaps as many as there are criminals.
First, we should note that homicide is not a new phenomenon. We have archaeological evidence for a murder that occurred 430,000 years ago. There is considerable space devoted to homicide in ancient law codes, something that is evidence of the ubiquity of this crime through human history.
Despite tremendous variation across cultures and periods, there are a few consistent things we know about homicide. First, across all periods for which we have data, the majority of perpetrators are male. However, correlation is not necessarily causation, and though people with Y chromosomes are more likely to commit homicide, one cannot claim for certain that Y chromosomes explain homicide.
Next, the perpetrators of homicide tend to be socially disadvantaged in some way. They may be people living in poverty, members of minority groups, or otherwise marginalized. There is also a solid correlation between homicide rates and GINI coefficient (a measure of economic inequality within a society). Thus one could also argue that inequality and social injustice tend to cause homicide. Of course, this sort of statistical analysis does not explain why one person may commit homicide and another not.
Interpersonal conflicts are a common motivation for homicide. The wide availability of guns in certain societies, such as that of the United States, also correlates with an increase in homicide rates.
There is never going to be one single reason for homicide. Normally, it is derived from many past experiences. Whether it be with that person they killed, their childhood, etc. Sometimes, the person actually has had something wrong with them since they were born.