I have always interpreted "Don't feed the trolls" as meaning "Don't reward bad behavior". It has never meant "Don't criticize bad behavior", "Don't report bad behavior", or "Don't fight bad behavior".
Like "hacker", the term "troll" has strayed from its original meaning. To me, a troll is someone who makes deliberately provocative and probably disingenuous statements in order to start online arguments, a.k.a flame wars.
Someone who makes threatening or intimidating statements in order to frighten people in the real world is more accurately called a terrorist. Trolls inciting a flame war are annoying; terrorists making threats of physical violence, either explicitly or implicitly, are criminals.
So, "Don't feed the trolls" can be good advice when dealing with people who just want to engage you in an unwanted online argument. Ignoring those people might not make them go away, but at least it will not further encourage them. But, it is not appropriate advice for dealing with people threatening or encouraging violence. Those people aren't trolls, they're terrorists.