>Hmm, what would make you assume perpetrators of violent crimes would have a different IQ level than other crimes?

For starters there's the lead exposure relation to violent crime, that is accepted as a factor, and which is also known to lower IQ.

That lead-affected criminal population would drive average violent criminal IQ down, even if the lead exposure worked through a different causual mechanism and lower IQ was just an orthogonal effect.

Besides several studies have found the general correlation.

>My initial instinct would be that violent crimes are often committed out of passion, and are unrelated to intelligence.

Choice of outlet for the outburst, impulse control and other factors however are related to intelligence.

Besides you're just covering "crimes of passion" here. There are career criminals doing homicides, gang shootings, etc, plus physical violence unrelated to passion, but related to intimidation, theft, etc.