Here's their reponse:
While we understand your frustration and the severity of your issue, we have found that the reported account is currently not in violation of the Twitter Rules (https://twitter.com/rules) at this time.
We have a policy against violent threats, but the content of this account lacks the specificity to meet the criteria of an actionable threat. While we cannot suspend or remove content for violation of this rule, it may be important to address the actual threat rather than the online posting alone.
We strongly encourage you to contact your local authorities to assess the validity of the threat. By contacting and working with your local authorities, you may be able to defuse the threat and address the root of the issue. Websites such as Twitter do not have the ability to investigate and assess a threat or apprehend and prosecute individuals.
If contacted by law enforcement directly, we can work with them and provide the necessary information for their investigation of your issue. You can point local law enforcement to our Law Enforcement Guidelines here: https://support.twitter.com/articles/41949
Keep in mind that removing the Tweet or other content makes it difficult for the threats to be investigated by law enforcement. Seeking suspension or removal of Tweets also may have the effect of encouraging the abusive behavior.
If you are dealing with a potential legal issue, please contact a lawyer. Twitter cannot provide legal advice. It is also a good idea to contact a trusted individual such as a friend or family member for support and advice. If you don’t have someone to talk to about what is happening online, there are many online resources that can help.
Twitter Trust & Safety
Twitter Trust & Safety
There are quite a few issues I have with this reponse.
Firstly, what kind of abusive message does Twitter consider to meet the criteria of an actionable threat?
Secondly, I wasn't asking Twitter to apprehend or prosecute an individual. I was asking them, as the provider of the platform this abusive person used to post such vile things, to consider suspending their account, or issuing them with a warning that such behaviour won't be tolerated.
Thirdly, is it me, or is this response basically suggesting that standing up to abuse will encourage it further? Because I know from experience that that's the case at times, but I certainly don't think it's a good enough reason not to stand up to it.
Basically, I remain pretty appalled with Twitter.