The trouble with this mantra is that it raises the question, Which Jesus? Whose Jesus? The Jesus of St. Francis of Assissi, the Berrigan Brothers, and Chris Hedges is not the Jesus of the born-again President George W. Bush, who participated in daily Bible study in the White House but casually started two wars. Yes, Jesus did in fact banish the money changers from the Temple--an act one can envision the non-Christian Bernie Sanders emulating--but he did not issue orders that resulted in the deaths of many tens of thousands of people. (If we ignore, as is commonly done, Luke 19:27.) So "WWJD" remains an insoluble problem.
Far less inconclusive is "What did Jesus do?" He brought to mankind the values enshrined in the Sermon on the Mount. These values are partly Judaic, partly original: Performing rituals is meaningless unless deeply felt; proclaiming morality is hypocritical if not accompanied with action; forswearing violence is the new definition of manliness and heroism. These spiritual mandates truly raised the bar on pious behavior, raised it so high, in fact, that hardly anyone has followed suit. As Nietzsche said, the last Christian was crucified at Calvary.
Nay, instead of improving human behavior, Jesus's message became the occasion and the excuse for worsening it. Consider the many early Christian sects with their bitter disputes over who are the true disciples of the Founder. Consider the violence that erupted among Christians over such matters as homoousian versus homoiousian, a dispute at once abstruse, insoluble, irrelevant, and absurd. Consider the Crusades, meant to liberate the Holy Land (itself more a Judaic than Christian concept), which degenerated into occasions for butchering Jews. (As in Mort Sahl's joke about Werner Von Braun, "I aim for the stars, but sometimes I hit London.") One of the Crusades was even about killing an exclusively Christian sect in the south of France. Or how about the Inquisition, addressed mainly to Jews and heretics in order to coerce them into the true faith, when Jesus emphasized that piety must come from faith within, not from mumbled words, above all not from coercion, as even Islam acknowledges. And as if all that is not bad enough, we have the great religious wars of the sixteenth and seventeenth centuries between Catholic and Protestant (and then in England among Protestants), wars fought by factions proclaiming themselves the true followers of the Prince of Peace. Wars in which killing was not enough but had to be preceded by torture in the name of Jesus. And then there is the Church either presiding over or blessing the vast atrocities perpetrated in South America.
What is going on here? The answer may be found in Monotheism, the supposed advance in Western Civilization. A conflation of remarks by Gore Vidal and Salman Rushdie is, One God results in One Truth, which in turn results in heads rolling. Voltaire first pointed out that the pagan societies with their silly polytheism had one powerful virtue that Christians lacked: They never fought a war over religion. Each people had its own god(s) and understood that the others were entitled to their different deities. Only Christianity was guilty of what Montaigne called supercelestial ideas begetting subterrestrial conduct. Granted, had Jesus never appeared, there would have been wars aplenty, but these would have been fought over the usual suspects--greed, power, lust, revenge, never over a person's private connection with an alleged numinous power.
Christians will, of course, rush in to say that one cannot hold Jesus responsible for what others did in his name, just as, for example, a gunmaker or car factory cannot be held responsible for someone using these products to deliberately kill others. But that is a poor excuse. By way of a response, two, and only two, possibilities present themselves.  That Jesus is, as proclaimed by devotees, God or the son of God, in which case His entry into the human narrative is part of God's plan to make human existence even more miserable with the aim of divorcing us from this world and throwing us into the bosom of God and the Hereafter. As these evils could not have taken place if the omniscient Jesus had not appeared, he must have favored what he brought about, or else why bother coming down from heaven? If that is indeed the project of the God of Love, He has turned "tough love" into a reductio ad absurdum.  Jesus is no relative of God nor any sort of supernatural being but, as Tolstoy put it in his later years, a simple human being with such profound insights that people felt compelled to assign divinity to him, at least metaphorically.
But the latter explanation results in the definitive condemnation of Jesus. He was a naïve bumpkin, a (to use conservative terminology) "bleeding heart" hopelessly out of touch with reality. He came to improve our lives, yes, but he had no understanding of how depraved human nature was ("original sin" was Paul's idea), of how beyond improvement we were, of how we would gorge ourselves on his ideas not to in order to live by them but in order to reinforce our basest instincts. He is therefore the classic example of how good intentions alone are dangerous, of the road to Hell being paved by them, of how pathetic it would be of him to declare, with Romeo, "I thought [=meant] all for the best." The fact remains that without Jesus's intervention, this world would have been a better place. However noble his message and pure his motives, has anyone--wittingly or not--caused greater harm? As for the hereafter, let those who believe in it take what comfort they can.