Much of the hatred we see does not have an obvious source. In some mass shootings, the killer has not even left direct evidence of what motivated him to start shooting at people. In these cases, we can only conclude that mental illnesses are often manifested in gun violence, more frequently than in the past simply because there are more guns now than before.
Conservatives often claim that mass violence is the result of the general "permissiveness" in US life since World War II or some other watershed event that is claimed as the end of the old order and the beginning of the new. In America today, however, several quite different factors have contributed to an atmosphere of hatred and violence, such as an overextended version of conservative ideology (rather than the "liberalism" conservatives assert is responsible for it).
As the US has become an increasingly pluralistic society, those who have traditionally been in power and been the dominant group now feel threatened. It is not necessarily racism per se that has caused this, although that is an element of it. It would be more accurate to label it xenophobia—fear of foreigners—combined with a fear that the previously dominant Anglo-Saxon culture will eventually be obliterated. The false and stereotypical perception of latino immigrants, bolstered by demagogic political rhetoric which became "normal" at the highest level in the last election cycle, has fed this. Some of it is, in fact, pure racism.
It's facile to say that Europe does not have the same problem because European society is not as pluralistic as America. Europe has become pluralistic since the period of decolonization. The more obvious difference is that in Europe, private gun ownership is relatively unusual. In America, which has been a frontier country until recently, guns have always been prevalent. It was partly this prevalence, already existing in the eighteenth century, that enabled the colonists to defeat the British and achieve independence. In Canada, the situation was different because neither the French nor the English Canadians rebelled against Britain but were granted independence as a Commonwealth country peacefully.
In the US, we hear arguments in favor of gun ownership repeatedly, based on the Second Amendment. Anecdotal evidence suggests, however, that the vehement defense of guns is rooted in a racist fear of a supposedly inevitable conflict that is predicted for the US.
Europe, on the other hand, in addition to not having private gun ownership as a normal part of their world, has already had its orgies of hatred and mass killing. After World War II, Europeans were so stunned by the mass destruction they carried out against each other that they seem to have eschewed mass violence, with a few notable exceptions. The latter include not only recent terrorist attacks but the savage and genocidal wars in the former Yugoslavia, where long-standing religious and ethnic hatred was never fully resolved.