Donald Trump has an obsession with revenge, and a veteran Washington, D.C., journalist warned that’s yet another reason his return to the White House would be an existential threat to American democracy. The former president has given conflicting statements on whether he would seek retribution for his felony conviction in New York, but Mother Jones journalist David Corn said Trump has long made clear that he expects payback against anyone who slights him. “Throughout his presidency, Trump condoned and encouraged violence,” wrote Corn, the publication’s Washington bureau chief. “And for decades…