Trump criticized mayor Sadiq Khan for the failure to curb the growing knife violence in London. https://www.cnn.com/2019/06/16/uk/donald-trump-sadiq-khan-london-stabbings-intl-scli-gbr/index.html

Notice Trump didn't just include homicide, here are some knife crime stats for London and Wales: https://www.bbc.com/news/uk-42749089

That part wasn't literal, it is more along the line of calling Republicans racist by blaming blacks for the USA's murder rate.