One could reasonably ask, "What if there really is a dangerous crisis; and our leaders really are corrupt, weak, or incompetent; and we really do need a strong leader? Shouldn't a candidate who points out those realities be considered honest and courageous, instead of a wanna-be dictator?" Possibly. If those conditions are real, then saying so may not differentiate a demagogue from a good leader.
The difference is what they propose to do about it. Fortunately, we have both good and bad historical examples to help us learn to discern. A great leader confronts challenges by building others up, emphasizing unity and shared values, appealing to our virtues (love, patience, humility), and reinforcing our commitment to fundamental principles of freedom and equality. A despot tears others down, emphasizes our differences to stir up contention (rich vs. poor, women vs. men, christian vs. muslim), cultivates a cult of personality around himself, appeals to our baser instincts (fear, greed, hate), and sacrifices fundamental values for political and economic gain.
Compare how past leaders faced crises to Trump's approach:
"We are not enemies, but friends. We must not be enemies. Though passion may have strained it must not break our bonds of affection. The mystic chords of memory . . . will yet swell the chorus of the Union, when again touched, as surely they will be, by the better angels of our nature." (Lincoln, First Inaugural Address)
"[T]he only thing we have to fear is fear itself—nameless, unreasoning, unjustified terror which paralyzes needed efforts to convert retreat into advance." (FDR, First Inaugural Address)
"The energy, the faith, the devotion which we bring to this endeavor will light our country and all who serve it--and the glow from that fire can truly light the world. And so, my fellow Americans: ask not what your country can do for you--ask what you can do for your country." (JFK, inaugural address)
"We welcome change and openness; for we believe that freedom and security go together, that the advance of human liberty can only strengthen the cause of world peace. . . . Mr. Gorbachev, tear down this wall!" (Reagan, Berlin, 1987)
And now, Trump:
"The U.S. has become a dumping ground for everybody else's problems. . . . When Mexico sends its people, they're not sending their best. . . . They're bringing drugs. They're bringing crime. They're rapists. And some, I assume, are good people." (speech announcing candidacy, 2015)
"The most important difference between our plan and that of our opponents, is that our plan will put America First. Americanism, not globalism, will be our credo." (convention speech, 2016)
"I will build a great, great wall on our southern border, and I will make Mexico pay for that wall. Mark my words." (speech announcing candidacy, 2015)
"I am your voice. . . . Nobody knows the system better than me, which is why I alone can fix it. . . . I am the Law and Order Candidate." (convention speech, 2016)