Generally, the Unreal Editor is a part of the Unreal Engine and comes with all modable UE games. The Unreal Engine 2 Runtime linked above is a barebone version of the UT2003 version of the engine and you are not allowed to use it for games. You can create levels, mod it etc, but the result will only work in the UE2 Runtime.
If you want to mod/map for a specific game, you need to buy that game and use its version of the Unreal Editor.