We would have to look at the past post history of both accounts to determine what went on here on Twitter.
The thing is that historically black people have been oppressed over the past several hundred years by white people through the process of colonisation, slavery, lack of human rights, and sadly this archaic attitude still seems to be in place.
"Kill all black people" was to some extent state policy for many countries until frighteningly recently - talking mid 20th century. My own country, Australia, is a horrible example here - Aboriginal Australians weren't even recognised as citizens until 1967.
All black people in the US only got the right to vote in 1965 with the Voting Rights Act signed by Lyndon Johnson.
An Australian politician, David Oldfield, this year said on a national TV broadcast talking about Australian Aboriginal culture "You just naturally let it die out. I mean frankly, it should have died out, like the Stone Age died out." http://www.smh.com.au/entertainment/tv-and-radio/first-contact-review-thank-god-for-david-oldfield-20161115-gspkr0.html
It's not at all about making it OK to say "Kill all white people".
It's about recognising that "Kill all black people" is a very real fear for many people, and working to make that no longer the case.