I think what I like most about ‘Legion’ is that it doesn’t really seem to care at all about comic familiarity. I didn’t read the stuff David Haller’s in, but it seems to me that the show is headed in a direction that makes him much more sympathetic and portrays him in a less, dare I say, ableist light.
I’m not the person to be definitive on this, but it kind of bothered me the comics seem to portray his powers as something that are inextricably tied to his DID, with each personality having control over a specific one. Psychiatric disorders happen because your brain tries to protect you, but in a way that harms you still. The show seems to show psychiatric disorders as something that makes you different, sure, but it necessarily doesn’t make you scary or uncontrollable or unworthy of fulfillment *cough* Split *cough*. You don’t have to be just like everyone else, but you do have to be cognizant enough to understand the way the people around you feel.
Making David’s illness the result of a parasite seems to be a more accurate portrayal of how people deal with mental illnesses (certainly it’s how I tend to characterize my anxiety and depression, even though it’s very, very different). He’s a victim, but he does have the ability to find an identity and fulfillment by going through this illness that he might not have otherwise. He’s sympathetic rather than pitied, and that makes a world of difference.