Just like a kitchen knife can be used by murderers does not mean that the kitchen knife manufacturer should actively monitor who is buying their knifes and go after the ones with bad intentions in my opinion. In the same way, I don't think free software should be licensed under such an ethical open source license.
First of all, it's just very hard to enforce I'd imagine. Would people who do not care about respecting human rights to begin with even care about such a license? You're not stopping "evil" by making up rules that "evil" people don't follow almost by definition. At least the license is more clear about what is considered as acceptable and what not unlike the infamous "The Software shall be used for Good, not Evil" clause, for which I have to give it some credit.
Everything you can make can be used for wrong things and I suppose you'll have to accept that you'll never have full control over that. The only thing that can stop something to be used by people with bad intentions is to never create it in the first place.
Besides, I´d rather have my enemy use open source software instead of closed source software. If they use open source software, it's easier to get a grasp on what they're capable of.
For software like Lemmy with federation, I'd just say to block those from federating to your own instances and stop giving those instances attention. As long as the flagship instances that federate with each other keep up a positive attitude, I would just ignore those extremist people's instances
It's not much different than a profile picture on most social media networks, is it? You could have done all of this with your Lemmy profile picture as well.