"Such theoretical shifts set the stage, for Berlin, for the ideologies of the totalitarian movements of the twentieth century, both Communist and Fascist–Nazi, which claimed to liberate people by subjecting – and often sacrificing – them to larger groups or principles. To do this was the greatest of political evils; and to do it in the name of freedom, a political principle that Berlin, as a genuine liberal, especially cherished, struck him as a ‘strange […] reversal’ or ‘monstrous impersonation’ (2002b, 198, 180). Against this, Berlin championed, as ‘truer and more humane’, negative liberty and an empirical view of the self." https://plato.stanford.edu/entries/berlin/

The idea of positive liberty is that you are free to become the best man you can be, but there is someone or a group who defines what 'best' is. And that inevitably ends in dictatorships or similar types of abuse.