RNG is when something is random. Him killing a target that is not high AD is not RNG, it is a decision and a cost-benefit analysis. However, it is almost ALWAYS benefit.
Let's examine this.
Zed hits level six and roams bot, where the opposing Sivir has been crushed and is easy fodder. He gets an easy kill and picks up her AD due to his new passive. He benefits, while setting Sivir farther behind.
Next, a few minutes later, he runs into Nidalee or some other AP champ that is farther ahead. He successfully kills them via outplay. Now, if they have a higher AD, he takes that into his new passive. If not, he keeps the AD from his previous kill.
If he must choose who to kill between them, and he picks their AP carry, while he might not update his passive, he still has made an impact and done his job. If he kills the weaker ADC, he might update his passive and be more useful overall, along with getting an easy kill and making it a 4v5. This is not RNG, but a decision that is impactful. [slayer-pantheon-thumbs]