[TeMPOraL]

The point of having the denominator is so that you don't need to assume anything, you can just know which way the rate is going.

Perhaps you're right that in this case, the rate isn't going to stay constant. But in general, with news reporting, I found it safe to assume that if the rate was rising, they would've reported on the denominator too - the reason to omit it is usually because it would kill the story.

EDIT:

To expand on this further - and again, I'm defending the original question as a perfectly legit one, not trying to diminish the fact that we're talking about murders here - you always want to see the whole fraction spelled out, numerator and denominator, because the other common trick to deceive with numbers is by providing only the ratio itself. E.g. "over past year, incidents of $crime rose by 80%", or "$substance increases your cancer risk by 300%", where the unmentioned denominator is "3" and "0.00001", respectively.

The way I see it: even if we're talking about clear tragedies or acts of evil, it's always better to have an accurate picture of what is being talked about.