> Compare that to the frequent truck/car ramming attacks in Germany which seemingly always kill tourists. Should I avoid visiting German Christmas markets?

Personally I would find such a risk acceptable just as I find the risk of traveling to countries with a high homicide rate acceptable.

But if a friend or family member expressed concern about visiting a German Christmas market due to prior cases of terrorists murdering tourists there I would not dismiss their concerns as irrational or unfounded as there exists a rational basis for this concern.