Perhaps this comes because a murder can be seen as a sad result of circumstances. Perhaps the criminal only had to murder because they had a bad upbringing and needed the cash and it was a horrible mistake. On the other hand, people perceived as racist are permanently marked as being ideologically dangerous. Mainly, I think this comes because racist tweeters can be seen as more directly responsible for their actions than a murderer.
Of course, this line of reasoning is nonsense, but it appears to be quite common.
Jon Ronson has a great TED talk about this, titled "How one tweet can ruin your life": https://www.youtube.com/watch?v=wAIP6fI0NAI. I would recommend that the grandparent post watch this.