Does a kilonova leave a high mass remnant?
I think it is generally expected that the merger of two neutron stars will lead to the formation of a black hole. What is more uncertain is whether that black hole forms straight away, or an intermediate state of a hyper(/supra)massive neutron star forms (see, e.g. Sec IIC of Hotokezaka et al, PRD, 044026, 2013 for a definition of hypermassive and supermassive neutron stars), which then collapses to a black hole after a short time. A few papers looking at the stability of such a hypermassive neutron stars formed from a binary merger are - 1, 2, 3 and 4 (check out the references and citations to them for more info). In the first three reference I think the collapse to a black hole is expected within a second (see Table 2 in Hotokezaka et al, PRD, 044026, 2013 for expected lifetimes before collapse times for various neutron star equations of state and masses), whilst in the final one a range of collapse times from fractions of a second up to several tens of thousands of seconds are found.
A search for a post-merger remnant from the binary neutron star merger GW170817 is now available here. In the introduction of this paper it describes in more detail some of the post-merger remnant possibilities that I mentioned above. The searches don't find any evidence for a signal, but given their sensitivities this isn't surprising (they'd require more than the total mass of the system to have been converted to gravitational waves to see anything!)