You make a point that we shouldn't just cave to any Jerk that comes along and used your code, which makes some sense. There is a silent user majority that appreciates these projects (or doesn't think enough about them). I'd advocate saying thanks, but if everyone did that might be a time suck for the developer, so maybe use the "star" if on github, or contribute if that is an option.
I agree with you that it is generally better to not to fold to these kinds of attacks.
However, the README.md does mention other reason why they would not really want to maintain it for sure anyways. Furthermore, the maintainer can make their own decision about what to do with it (whether or not other people agree with it).
And, since it is FOSS, it is not much of the problem if the maintainer decides to stop maintaining it, since FOSS is possible to fork the project and other people to work on it too, anyways.
This is not an excuse for writing such a bad email message, but nevertheless someone might do so and therefore you will have to decide how to deal with it. (I think it is best to ignore it, but of course everything will influence anyone)
To be clear, I don't want to sound callous or say that this is, in the long term, the cost of doing business in open source (it absolutely should not be); but if you're a person with a publicly-routable email (say, via git commiter or author metadata), on a long enough timeline, someone awful's liable to find it and use it maliciously.
My opinion (admittedly, not having been on the receiving end of something this pointlessly awful) is that it's better not to fold to these kinds of attacks. These people are basically schoolyard bullies, and usually attention-seekers. Validating their attempts only encourages more behaviour like this.
I hope the former maintainer is doing well, and I hope this message doesn't come off as disrespectful or harmful. If it does, I'm very open to hearing about alternative approaches.