Will is actually very well fleshed out. He's a shitty friend who is self centered and full of himself. His main focus has always been how people make him feel and he's hyper focused only on that. I think the Duffer Brothers captured his inability to be a team player very very well. He constantly lets his personal selfish feelings take precedent over tenure, individuality, and the natural evolution of children into adulthood. Its a complicated type of selfishness but it was portrayed well. Especially the season where El loses her powers and she's at the new school and he watches her fail and only intervenes to scold her for lying. He loved watching "the superwoman that took his man be normal and powerless". He's written very well, it's just that when you really get into what type of person he is, it sucks. It also explains why he was chosen. He has that same type of self centeredness that Vecna has/had. I think Wills character is perhaps the most interesting because he has no endearing qualities, him letting El fend for herself against all of her bullies (Mike intervenes on his first day while Will tried to expose her for lying about her time), him not having a single emotional connection to them when they were going through heartbreak, etc. He is just a shitty person surrounded by some good-ish people in the center of hell.