Alright, let me summarize the case against Nikells:
Once hatmoza turned on him, he's spent the vast majority of the game hiding; he's only recently reappeared, and it took people voting to lynch him for that to happen. In other words, he's been lurking for much of the game, even though he made himself fairly high-profile early on.
He's claimed to be a doctor, who can heal every night. I'm a doctor, and I can not act every night, so it's somewhat discongruous. Moreover, if I recall correctly vetteman said that once he found Morpheus he (vetteman) would become a doctor, rather than a regular townie: if that's true, it would result in the town having three doctors, which seems a bit extreme.
He acted incredibly cagey about his role, long past the point where most townies would: while it's understandable that a doctor won't come out and say he's a doctor, time basically ran out and he still hadn't bothered to role-claim and try to save himself. Considering how rare it is for people to be online simultaneously, and how he was the only person with votes on him at the time, it seems unwise for a doctor to risk dying in order to, er, not risk dying. Add to this the statement that he no longer cared as much about dying as he did on Day Two: apparently, doctors become less useful as the scum have fewer targets?
Also, he attempted to shift the town's attention away from hunting mafia and towards hunting the cult, to the point where he came up with reasons why the cult was as dangerous, if not more so, than the mafia. He's also displayed a shocking indifference to important things like the God Of War series' storyline, in spite of there being heavy discussion throughout the game as to what, if anything, roles mean. Strangely, this indifference did not extend to making sure that his own name could not be affiliated with scum, which is definitely an interesting distinction. Finally, he's name-claimed someone who's not in the series: the topic's previously arisen, but unlike TruckOSaurus he didn't speak up at that time.
Am I missing anything?