Earlier hits were using Ukrainian drones, while the Atacms are reported as needing to be programmed by US military to hit the targets. So while it is Ukraine that supposedly fires them, it is the americans who will reportedly get them to their intended targets. I don't think there is any moral debate in Ukraine hitting Russia with missiles. After all it is a war fought by Russia against an Ukraine which has Nato proxy support. But it is an escalation nevertheless.
It is now up to Russia on how to respond. And as you noted, one scenario being talked about, at least in social media, is some groups houthis, hezbollah or others getting Russian missiles and those being fired at western targets, ships or others. And I assume it would be Russian military who would control the targetting in that case depending on the missiles used. Or the Russians don't go for direct escalation with the intent of not jeopardizing the chances of Trump ending support to Ukraine in few months from now.
But either way Russia's deterrence against Nato has been challenged yet again, and the chances of escalations and counter-escalations going out of hand remains a more nearer scary possibility in the unfolding scenario in process.