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Occasionally when people find problems or design decisions in your project or product that they disagree with, they will deliver this message to the project in an aggressive way (reasons for this may involve that digital communication make blunt tools, language barriers and more). Do not be surprised to read insults and insinuations about not knowing what you are doing even if you have worked on the project for ten years and this bug reporter tried it for the first time yesterday. One of the hardest things to do as a maintainer is to bite your tongue and answer politely and friendly. You will not gain any bonus points nor any new friends by lowering yourself to the attacker's level and (try to) deliver insults back.
You will learn that you have chosen the wrong language (no matter which language you use), the wrong technology (either it is too old, too stupid, too clever or whatever that can be wrong), that your software does not solve the problem the user has (even if it never was meant to do what the user wants it to do or if the person simply has not understood how to make it do it). And you will get told this in a hostile way.
As you want your project to ooze of friendliness and cooperation, you do not want a sour message from a project maintainer to remain in the public to scare off future users or contributors. Keep a good tone when responding, even to those who have done nothing to deserve it.
This way of delivering a message is bad and wrong, but you should still be prepared that it will happen.
Take a deep breath. Do not write that email if you still feel upset. Consider that the author of the tool most likely did their best even if you found a problem. They may have not considered your use case. They spent countless spare time hours and shared their code in the open for everyone to use.
The least you can do is to be friendly and courteous when communicating. It also goes for when you report bugs and even if the people in the other end snap back at you or seem to imply that you are stupid. You can take the high ground.