Previously you could tweet "kill all white men" and nothing would happen. Not so much with the same statement targeting a different group.
https://en.wikipedia.org/wiki/Bahar_Mustafa_race_row
http://i.imgur.com/iojbGhE.png
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http://i.imgur.com/8vg96Cz.png
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> However, on 26 October the Metropolitan Police dropped the charges, revealing that they had discontinued their criminal case against Mustafa because there was "not enough evidence to provide a realistic prospect of conviction".[25][26][24] Under the Victim's Right to Review Scheme, one of the complainants in the case then requested that the Crown Prosecution Service review their decision to terminate criminal proceedings.
#killallmen is not meant as a serious threat against any one person, or even a group of people. It is meant as an—albeit radical—battle-cry to topple the patriarchy. At least that is how most courts and most moderators see it, so censoring it like if it were a serous death threat would always be rather silly.