

In that context it sounds like they are using the word “sadly” in place of a word like “unfortunately”.
“unfortunately, i’m referring to you.”
“Unfortunately, i think you should suffer and realize what an annoying piece of shit you are.”
Now, as to why it’s sad or unfortunate, I’m not sure. It could be that the person saying “sadly” likes the other person but is upset with them in the moment, or the person saying “sadly” could be trying to soften the blow of a mean comment like the one you used in the example, or they could be saying it’s sad/unfortunate for the person they are talking to.
Just my two cents.
We said “Thoughts and Prayers” that’s obviously enough.