From Death Row to Throne

Liam is an assassin at the top of his trade, and he wakes up in the distant past on death row. What he has there is a system that makes him stronger with every enemy he kills. He gets himself into the army and rises through it, killing rich men who have been abusing their position, which turns the imperial court against him. To get out of the standoff he fakes a defection to the other side and works with Eleanor, a general. He beats the enemy commander in the field. The victories are large enough to frighten the sitting monarch, who is incompetent and moves to have him framed for treason.