In short, it's getting safer to be a cop, and more dangerous to be a criminal.
There's something to cheer about here - in general, a society might prefer to have living cops and dead felons, rather than vice-versa. But there's a good deal to worry about, too.
This year there have been numerous high-profile tragedies involving cops shooting unarmed civilians. Just this past week, a 12-year-old boy brandishing a pellet gun was killed by police in Cleveland in broad daylight. In New York last Thursday, cops accidentally shot an unarmed man in a stairwell. The specter of Michael Brown looms over it all.
The decrease in officer deaths and rise in felon deaths has corresponded with a rise in the militarization of the nation's police forces, fueled by a glut of surplus military equipment heading home from Iraq and Afghanistan. Few people would object to the outfitting of police officers with better body armor and protective gear. But the assault rifles and armored vehicles, so visible during this summer's protests in Ferguson, are a different story altogether.
Smart policing requires an element of trust between officers and the community they serve. But in some cases it appears that militarized police forces are operating under a motto of "occupying" cities and restraining the population, rather than protecting and serving. As we've seen in Ferguson, that only heightens tensions.
It's particularly worth noting that the FBI data on justifiable homicides is widely understood to be substantially undercounted -- some states don't participate in the FBI's data-gathering programs at all, and others don't tally justifiable homicides separately. So while the figures above are useful for generating a trend, the actual national numbers are considerably higher.