If you need to murder thousands of people to legitimate your regime, your regime ain't legitimate.
Speaking as someone who knows people on both sides:
It's going to be very hard to convince most iranians that this is anything other than Israel/USA backed terrorism. Especially with Israel and the US openly bragging about backing it.
It doesn't help that they're probably mostly right. And looking at the genocide in gaza and how many of the people backing the protest are basically holocaust deniers, it looks really horrible to much of the islamic world.
To quote the current US administration - One of ours, All of yours