If you're looking for a Champion that racks up kill counts, then Mordekaiser isn't for you. He lacks the crowd control to really chase enemies and finish them off. This isn't to say he won't get first blood or any kills, he just has to work harder than most have to. Mordekaiser's strength is not the fact that he can kill champions, it's the fact that he can recieve punishment and give back quite a bit a punishment thus pushing the enemy back. He's very good at early game champion harassment with his Siphon of Destruction, and mid to late game he can act as a spearhead for charges. His Ultimate, if used in the correct manner, can get you a very early turret kill. His Innate allows him to shrug off most damage and effectively ensures he stays in the game with minimal HP loss.
Most people underestimate Mordekaiser because he's a tank without CC that does minimal damage, ergo they go for the squishy standing behind him. You can make people pay seriously for doing this.
Overall, Mordekaiser fulfills the role of a tank very nicely. His usefulness, IMO, stays consistent throughout the entire game. A late game Mordekaiser can essentially make or break a game. However, if you're looking to rack up kills, then you're better off with Master Yi, Twitch or maybe Garen.
Given that this was a topic pertaining to Mordekaiser, I stuck mostly to Mordekaiser description, hopefully it gave you a little insight into what Champion you're going to choose.