It might be more fruitful to put the other way around and ask 'Does a colder climate and reduced daylight hours reduce violence?' In Greenland, where days extend to 24hours in the summer, violence increases. Or to put it another way, look at countries that have more extreme variations in environmental conditions in terms of light and temperature throughout the year to try and see patterns. It may just be that amenable weather and daylight hours encourages a higher rate of social interactions and, hence, potential for violence.