The Sun reported that this month has been the fifth deadliest in 40 years.
Actually, by rate, since Baltimore has fewer people than it used to have, May has been the most deadly month ever.
In number of dead, the deadliest months have been:
Aug 1972: 45
Dec 1971: 44
Aug 1990: 42
Aug 1996: 39
May 2015: 42
But the homicide rate (per 100,000) for these months are, in rank order:
May 2015: 6.7
Aug 1990: 5.7
Aug 1996: 5.7
Aug 1972: 5.0
Dec 1971: 4.8
And it's worth pointing out that May isn't over yet. (Also, May isn't August, the usual deadliest month.)
Also, those homicides rates are for one month and still higher than the yearly national rate. Put another way, even if no other people had been murdered in Baltimore before May, and even if no more people were killed from today until 2016, Baltimore would still have an above average annual homicide rate just based on the May killings.
Population figures are here.
UPDATED June 4, 2015