These are the three Darius builds I have found that work the best for an aggressive kill hungry Darius player.

Building Darius:

You start off with your standard lvl 1 boots and health pots. I have found out that getting a mana pot helps when you are poking a lane with your "Q."
The reason I get cooldown boots over any other boots is because Darius runs off cooldowns. The more Devastates he can do and the more Apprehends he does will help out the team and rack in those critical kills.
The reason I build Frozen Mallet first on all of my builds is because you get a little damage and a lot of health, plus the slow is nice too. The other reason is to test the other team. By the time you have built Frozen Mallet you will know if you can go pure dmg, a little beefy dmg, or tanky dmg.