How can I go there if I already killed Maliketh?

How do you access it in Capital of Ash?

You can still access the colosseum if its become the Ashen Capital. To do so you need to head to the big dragon corpse and traverse over it's wings and just follow the walkway.

To access Leyndell Colisseum in the "after" event:

-Leyndell Capital grace.
-Run W to the dragon corpse.
-Head slightly South-ish and you'll see a part of the dragon that digs into the ground.
-Run up the arm/wing till you can see art of the building with a ladder.
-Jump to the area.
-Go up the ladder.
-Continue the path onwards and it'll take you to the Colosseum area.

Strange that it only allows duels. Might have been interesting to also have the FFA, and Team modes without respawns. Or FFA with some powerful NPC or creatures as well trying to kill everyone.