Because it's common in shonen that one of the mentors might be a traitor; and since ONE was pushing so hard the idea of Metal Knight being a traitor, it just felt fishy for me. It would have been a big plot twist if suddenly the character that is not the one you first think of, happens to be the real enemy... it went down to Kuseno and Child Emperor, Kuseno would be the bigger surprise, so I stick to him.
"Kuseno" (クセノ/駆世野) in Japanese, particularly known for the One-Punch Man character (Dr. Kuseno), is a name that functions as a pun.
Pun (Kusai - 臭い): The name is a pun based on the Japanese word kusai, which means "stinky" or "odorous." It's pronounced "Kusee-no" (くせーの), which sounds similar to a colloquial form of "this sucks" (kusee nou), indicating a "suspicious" or "stinky" quality.
Literal Meaning:
Kuse (癖/くせ): Habit, vice, or peculiarity.
No (野): A common suffix in Japanese surnames meaning "field."
Context (Dr. Kuseno from OPM): Often interpreted as an indication of the character's suspicious nature or a reference to him being "stinky."
In short, Kuseno translates phonetically, but its etymology suggests "stinky field" or an insinuation of being "suspicious" or "shady."

Let's be honest...
You guys really didn't think that Kuseno might be evil? I had that theory since day 1. I still hold on that idea, I mean is not 100% proof until ONE draws it on the comic, so we might have to wait a little, but I really thought he was an enemy since years years ago.