What does dreadful but nesscary mean?

"It is dreadful but necessary" ("Cest affreux mais nécessaire"), from the Journal d'Autre Monde, 1794. ... This was a commonly held belief in 1793-1794 when the guillotine was a means of purging France of those who were deemed a threat to national security.

I think dreadful means something in dispersal or negative feeling like for example "his last words were very dreadful for the family".

answer; taking up arms to defend the nation;