1- In my opinion, there is no such thing as a "threatening meme." A meme cannot physically harm me or my property. However, I do admit that there are "politically incorrect" memes ofensives, but it's important to clarify that this incorrectness depends on the political stance of the person viewing it.
If A has a political stance opposed to B, then B will likely not have a major problem with the memes that A considers "politically incorrect"—memes that A sees as ofensives to their religion, culture, etc.
That said, I cannot agree with considering a meme "punishable" in a country like Venezuela, where having a meme on your phone that contradicts the dictatorial regime in power can lead to unjust imprisonment.
2. When is it acceptable to threaten? I believe never. There are memes that, based on my political or religious stance, I approve of and agree with; I share them. A little dark humor doesn't hurt anyone. But there are other memes that are reprehensible, and I consider them not just threatening but offensive. In that case, I would never repost something like that.
For example: the audacity of the French magazine years ago when it published the photo of the child on its cover. I didn't consider it a threat, but it was highly offensive.