"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.
"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.