Ah, Ryme City: A beautiful place where people partner with Pokémon to create a utopia for all beings. The city is the idea of Howard Clifford, CEO of a huge corporation and creator of advanced hologram technology. Mr. Clifford has a degenerative disease and uses a wheelchair because of it.
But, someone has nefarious plans for the people and Pokémon of our lovely haven. Who would destroy something so amazing? Howard Clifford, of course! Why? Because human minds taking over Pokémon bodies is the next step in evolution. He has found a cure for human frailty. He has found a cure for his disease.
Howard wants a cure with such intensity, he's willing to destroy everything (even his relationship with his son). We don't know how long he's had his disease, how it impacts him (besides the wheelchair), how old he is, or what his prognosis is. Without more information, all we get from the screenwriters is some variation of "disability bad, must fix". I'm honestly tired of seeing a "cure" as one of the only outcomes a disabled character could desire.
Howard is rich. He has a whole city designed by him, a team of scientists under his command, technology most folks only dream of, and people who adore him. He spent so much money, his scientists discovered a way to merge humans into Pokémon (that couldn't be cheap). Was it truly easier than them finding a pill to slow the disease or an injection to reverse it? I have doubts.
If Howard wanted to heal, why drag everyone else into his scheme? He didn't just try to transform the disabled, the elderly, or the dying into Pokémon... his plan included everyone. The vague mention of "evolving" isn't an answer I accept. Maybe he could control people if they were Pokémon because he transferred his mind into the greatest Pokémon ever (Mewtwo). But, world domination is an entirely different motive.
Was he a good person before he wound up in a wheelchair? The movie doesn't say. Maybe Howard was a bastard his entire adult life (his son is a good person, so maybe the mom raised him). The only thing we really know is that he's absolutely desperate for a cure, so I would think it's the disease driving him to unscrupulous acts. Disability made him into someone else... an evil someone else.
The game the movie is based on has a different antagonist. Howard Clifford could merely be a poor attempt to subvert the audience's expectations. However, with Hollywood only giving out a few worn-out tropes to cripples, we can't say the bitter, desperate-not-to-be-a-gimp villain was a genuine surprise. At least the Pokémon were cute.