Even if it is a purely cultural issue, completely separate from the religion of the perpetrators, we wil not be able to identify this conclusively unless reports on honour killings actually list the religion of the perpetrators.
What you're saying is equivalent to requiring the religion be reported of any criminal. We won't know that there isn't something common in the religion of most people who deal drugs unless we collect that information. Or the religion of most people who violate copyright laws.
While we're at it, why not also report their income levels? It often plays a bigger role than religion in many crimes.
And perhaps how many kids they have.
And on and on.
First, most perpetrators of honour killings justify their actions on the basis of their religious beliefs.
Citation needed.
Second, if it turns out that it honour killings are the result of a cultural factor independent of the religion of the perpetrator, reporting the religion of all perpetrators of honour killings would help dispel the impression that it is primarily a Muslim issue.
AKA the "If you've got nothing to hide, you should allow us to search your vehicle" argument. When various studies have pointed out it's not a Muslim issue, and have pointed out it's a cultural issue in a few geographical regions (or by people from those regions), the need to show this is lacking.
You know what's a big problem? Guns and Christianity. Why is it that most gun related homicides in the US involve Christians? I think that the religion of the perpetrator should be reported whenever there's a gun related homicide. Only that way can we dispel the notion that it's not a Christian problem.
Of course, you could point out that a lot of countries with large Christian populations don't have much gun related homicides, but I'll conveniently ignore that just as you would ignore predominantly Muslim areas where honor killing is not an issue.
Going off on a tangent below:
And, you know, it's a fairly pointless discussion until you state what you mean by "honor killing". Many examples given often involve jealousy or simply "looking bad in the society" (boyfriend feels he is being made fun of) - that's not particularly rare in the US (or often in Latin American countries) amongst non-Muslims - yet they're not defined as honor killings.
It's also fairly silly to make honor killings seem like a crime worse than your usual run of the mill homicide, which far outnumbers honor killings. When the US has a significantly higher homicide rate than many countries where honor killings are "common", why is it that the focus is always on the latter? Shouldn't we, according to you, report the religion of anyone who commits homicide in the US? Reducing the homicide rate in the US to that of Turkey, Jordan or Lebanon would be a greater feat than if any of those countries eliminated honor killings. So why does the latter warrant special treatment - even when such a crime occurs in the US or Canada?
If it happens in the US or Canada, at the end of the day, the only relevant issue is whether the justice system found them guilty - just as it is for all homicides.