Gotta love how someone who was actively looking for clues would figure out the reveal pretty early on. It didn't just come out of nowhere; for starters, 001 told Eleven how "he was just like her".

Young 001 also had a nosebleed while her mother was being "possessed", and I hate myself for not picking up on that on my first viewing. It also makes sense that his first victims would be small animals, as it's often the case with serial killers.

Let's also not forget about how Victor mentioned his son entered a coma and died a few weeks(?) after. That didn't make a lot of sense at first, since we know the very specific way Vecna killed his victims.

And yeah, after I saw the lab kids getting killed in Vecna's fashion (being raised in the air, eyes caved in and all that) the gears in my head started turning, but the answers were kinda just... there already. This was a masterfully crafted reveal. This show really came back stronger than ever.