I often build Hydra as a first item on Zed as well; it sure beats a BotRK Zed in a mirror match up.
Zed's abilities all scale off AD, and Hydra has much more of that than BotRK. Additionally, the active on Hydra acts as an AA reset, temporarily allowing Zed's next two AAs to be as quick as two AAs with BotRK.
BotRK's passive is great and all, but theoretically, it is inferior to other high AD items such as Hydra or BT until the enemy has over ~1000 health. The active can be used along with Death Mark for extra % health damage; however, it is still physical damage which is blocked by armor. As well, Zed does not need the slow as he already slows his target with a shadow E.
Hydra also has a much more useful build path than BotRK, which only comes into play once it is completed.
I still build BotRK on Zed, albeit not the first item because it isn't necessary. Hydra lets me score kills with ease in lane because of the consistent AD and it makes my poke much stronger. (92 more damage than a BotRK Zed with a standard W, E, Q combo)
