  Motivated by recent reports of metallic behavior in the recently synthesized
RuI$_3$, in contrast to the Mott-insulating nature of the actively discussed
$\alpha$-RuCl$_3$, as well as RuBr$_3$, we present a detailed comparative
analysis of the electronic and magnetic properties of this family of
trihalides. Using a combination of first-principles calculations and
effective-model considerations, we conclude that RuI$_3$, similarly to the
other two members, is most probably on the verge of a Mott insulator, but with
much smaller magnetic moments and a strong magnetic frustration. We predict the
ideal pristine crystal of RuI$_3$ to have a nearly vanishing conventional
nearest-neighbor Heisenberg interaction and to be a quantum spin liquid
candidate of possibly different kind than the Kitaev spin liquid. In order to
understand the apparent contradiction to the reported resistivity $\rho$, we
analyze the experimental evidence for all three compounds and propose a
scenario for the observed metallicity in existing samples of RuI$_3$.
Furthermore, for the Mott insulator RuBr$_3$ we obtain a magnetic Hamiltonian
of a similar form to that in the much discussed $\alpha$-RuCl$_3$ and show that
this Hamiltonian is in agreement with experimental evidence in RuBr$_3$.
