Greetings:
I don't think it's as much as Russia "choosing" to fight KSA as much as being forced into it.
The genesis of the latest brouhaha is the result of the breakdown in three way talks between KSA, Russia, and OPEC with respect to the cutting of production in response to covid-19. Russia as I'm sure you're aware isn't an OPEC member, but has been allied with KSA and by extension OPEC (loosely called OPEC+) for four or five years now to keep production at a level which will prop up prices. The agreement helps Russia project power in the ME and to a lesser extent globally (their MO for the past several years). You're right in previous comments that Russia needs oil at a certain price level to be profitable, so obviously they weren't interested in OPEC cuts. Furthermore, in order to have this "membership" in the first place, they were already agreeing to a ~2m bpd cut. I think that agreement expires soon anyway, so unless they can sort this out, Russia can easily grab the "excess" production capacity that the market will bear.
This also happened in 2014, but ultimately the costs were too much to bear. KSA doesn't have much intent of sticking it out, they're more interested in causing price shocks. They've raised output above their refining capacity, which means they are tapping into their storage. I don't know what storage numbers look like, but obviously not sustainable in perpetuity.
As for the U.S. shale industry? Well, that's either an unintended side-effect or the real target here. Either scenario is plausible. As stated in previous post, U.S. shale companies had been finding it difficult to raise capital through equity investment and so had to turn to debt financing instead. And now that's drying up. If oil prices are below that magical number required to be profitable, "poof". They be gone. So there are a few dynamics in play here. One dynamic I'm not sure about .. how does KSA's game plan change in light of the Saudi Aramco IPO?
Also worth noting, this is more than just covid-19 curtailing demand. Global demand for oil will be shrinking in 2020, which compounds the current problem.