Murder should be easy to measure. Unlike many other crimes or rights violations, the evidence is clear and painstakingly recorded: there is a body, at least in most cases; police seldom fail to notice a murder; and relatives or friends of the victim rarely fail to report the crime. So even if we are not always able to find and punish murderers, we should at least know how many murders there are.
And yet, even this most obvious of crimes can be hard to measure. In poorer countries, police departments may not have the means necessary to record homicides correctly and completely. Families may be weary of reporting homicides for fear of corrupt police officers entering their homes and using the occasion to extort bribes. Civil wars make it difficult to collect any data, including crime data. During wartime, homicides may not be distinguishable from casualties of the war.
And there’s more. Police departments in violent places may be under pressure to bring down crime stats and may manipulate the data as a result: moving some dubious murder cases to categories such as “accidents”, “manslaughter”, “suicide” etc.
Homicides usually take place in cities, hence the temptation to rank cities according to homicide rates. But cities differ in the way they determine their borders: suburbs may be included or not, or partially, and this affects homicide rates since suburbs tend to be less violent. Some cities have more visitors than other cities (more commuters, tourists, business trips) and visitors are usually not counted as “population” while they may also be at risk of murder.
In addition, some ideologies may cause distortions in the data. Does abortion count as murder? Honor killings? Euthanasia and assisted suicide? Laws and opinions about all this vary between jurisdictions and introduce biases in country comparisons.
And, finally, countries with lower murder rates may not be less violent; they may just have better emergency healthcare systems allowing them to save potential murder victims.
So, if even the most obvious of human rights violations is difficult to measure, you can guess the quality of other indicators.