Questions have been raised about his 3rd GM norm, but the jury's still out on that. It wouldn't surprise me if he has cheated in OTB as well in the past, given his shady history and behaviour.
But Carlsen really needs to grow the fuck up. As GM Ben Finegold pointed out, withdrawing from a round robin is a dick move, and it kind of ruins the tournament because players can be forced into a draw in a critical round where they need a win. In addition to that, passive-aggressively withdrawing with a cryptic tweet is not even close to the proper etiquette in response to suspected cheating. And it doesn't even make sense from a chess standpoint. He wasn't going to play Hans again. Withdrawing doesn't give him back any rating points. He obviously had no concrete evidence or Hans would be disqualified, so it's not like he's saving the tournament for the other players. He's just being a self-important jerk.
I'm glad he's declined to defend his title because he's no longer capable of being an ambassador for the sport. He only cares about himself(which was basically his explanation for declining) He's also gotten in bed with illegal gambling sites, trying to turn the Norwegian Chess Federation into a lobby organisation for the gambling industry(luckily he failed).
I say this as someone who grew up idolising Magnus Carlsen. I did 2 years ago, even 18 months ago. Not anymore.