THE LAST CLIENT did not get a theatrical release in the US, but the film is available for rental on Amazon Prime.

British novelist and film critic Kim Newman provides a useful synopsis:

Novelists Anders Rønnow Klarlund and Jacon Weinrich (who publish as A.J. Kazinski) wrote this twisty screen original, and [Anders Ronnow] Klarlund directed it. Though third act (and a very significant coda) get out of the luxurious office for the wilderness and some exceptionally tricksy plotting, the bulk of the movie is set during Susanne’s hour of talk therapy with Mark … though even the apparent premise –a Sopranos/Analyse This twist with a serial killer seeking psychiatric help–is deceptive, as Mark needles Susanne, spending more time digging into her past than his own. Eventually, we realise that Mark may not even be the kind of monster we took him for, but another, possibly even more terrifying human fiend with a long-laid, Mission: Impossible-level plan to get specific revenge on Susanne, for reasons that gradually spill out.

[snip]

Here's the IMDB link:

https://www.imdb.com/title/tt17595262/?ref_=nm_flmg_wr_2