Chaos: Uncharted Hearts (Uncharted Hearts, 3) by Constance Fay

Lore Olympus meets Winter's Orbit in this pulse-pounding romance between a space mercenary and a super soldier whose mind-control breaks when she touches him.

He's a mind controlled killing machine, until her touch frees him

Engineer Caro Ogunyemi thinks she has everything in control. Sure, she has a dark secret in her past and aim so bad that she can’t shoot the side of a spaceship when she’s right in front of it, but those are minor details in the life of a space mercenary. When Caro embarks on a solo mission infiltrating a prison planet that is run by the deadly Pierce family, she embraces the opportunity to prove she’s a hero.

It's there that Caro meets Leviathan, a super soldier with a chip in his head that turns him into a mindless killer. He’s drop dead gorgeous with an emphasis on drop dead, until she touches him and renders his chip inert. The danger begins when she lets him go.

In the heart of enemy territory, where love is at stake, life is treacherous and time is short, Caro and Leviathan must figure out how to recover his agency, protect her crew from Pierce’s sinister machinations, and stage a prison-break before Leviathan is lost to her―and himself―forever.