I am writing a report about someone and would like to convey the sense that he was desperate for success and overly ambitious. I've used phrases like "stop at nothing" to capture the ambition and the willingness to do set aside morals and/or break the law. I would like to use different expressions as I think "stop at nothing" is a bit cliche.
There are a few questions on this site with helpful terms including this one: "Steamroller" to describe a person as very determined
"Unstoppable" is very straightforward
"Relentless" is good too as is the related "unrelenting"
Since the OP refined the definition in the comment, below, I think "driven", "irrepressible" and "ruthless" work very well also.