You just hear words and phrases and don’t understand the historical context behind them

That's the thing, "rest in power" was not something that originated the way you are claiming. In recent years it certainly has often became more associated with that, but that is not where it originates.

You never would have seen a single person making this criticism in 2005 (when it had no association with what you say it means), or even in 2014 when some used it for a trans teen then Leelah Alcorn, when it had a loose association by that point).

It's sad that people are criticizing others mourning her based on what words they decide to use while there are still people openly celebrating her death.