Even though Cruel City is about undercover cops it's also about going up the hierarchy of the underworld and getting revenge. Last is also about climbing up the ladder and also about revenge. But most importantly, both dramas will make you wonder what is good, what is bad, what is the right thing to do and what is justified,
Both stories generate from the main characters orchestrated downfall, which forces them to seek revenge on those responsible. However, although it's understandable, they at some point become just as corrupt as those they are seeking vengeance upon. Ultimately, they have to choose... justice or injustice.
Both have gangster story lines and deal with themes like loyalty and revenge. Both have young leads who find themselves down on their luck and trying to start their lives over, this time in the underground. I also think that Park Sung Woong and Lee Bum Soo give off similar vibes in their portrayal of the bosses. Last feels like a more fleshed out version of For The Emperor (and I enjoyed it more).