Donald Trump tweeted some made-up statistics which coincide with his scary world view, but which are entirely fabricated. In actuality the percentage of whites killed by blacks is 14% compared to the 81% shown in his tweet.
Think Progress has a good analysis and here is a link to the actual FBI data from 2014 shown below.
Think Progress has a good followup post here on the media response
Daily Kos has a much more detailed article with more accurate stats as shown below
Election Diary 