So I was reading the Staff of Change description, and noticed that it says "This weapon in?icts 1 random mutation on the target if the attack generates Zealous Hatred [...], but if the psyker fails the Focus Power test, then the wielder gains 1 Corruption Point instead.".

So, which Focus Power test would that be? Is it implied that you have the option, just like with the standard "Killing Will", to test Focus Power to mutate someone when you inflict ZH, but get 1 corruption point if you fail the test?

Or maybe you must test Focus Power when you inflict ZH to mutate, and suffer corruption if you fail, like it's a backlash from the staff's mutation attempt.

Or maybe it's any Focus Power test while holding the staff (which would be a real big bother)? Not what I personally think, considering the wording with "the", instead of "any".

How do you guys interpret it?

While it's awkwardly phrased, my "common sense, let's make this work easily" reading is the following:

When a hit with a Staff of Change generates ZH, targets gets one random mutation.

When the psyker fails his focus power test to use killing will, he takes 1 corruption point as a failing.

These two special characteristics are not inherently connected. You can generate ZH without killing will and you can fail killing will tests without generating ZH. They stand alone, though they might occasionally occur simultaneously.

I agree with Reverend. It's the most reasonable interpretation.

I also see the sense in Rev's interpretation.