One, he is talking about the long term. The trend lines are uniformly positive across the centuries and mostly positive across the decades, but go up and down year to year...
The second caveat is both more subtle and more important. In 2013, I wrote about how technology empowers attackers. By this measure, the world is getting more dangerous:...
Pinker's trends are based both on increased societal morality and better technology, and both are based on averages: the average person with the average technology. My increased attack capability trend is based on those two trends as well, but on the outliers: the most extreme person with the most extreme technology. Pinker's trends are noisy, but over the long term they're strongly linear. Mine seem to be exponential.
When Pinker expresses optimism that the overall trends he identifies will continue into the future, he's making a bet. He's betting that his trend lines and my trend lines won't cross. That is, that our society's gradual improvement in overall morality will continue to outpace the potentially exponentially increasing ability of the extreme few to destroy everything...