For Kant, are we obligated to treat people who have no ability to reason or to give decisions or consents (concerning with mental insanity) as always an end and not merely as means? If we are, why is it? And why we wouldn't do the same to animals who are sentient?
In his written works, Kant did not write about such human beings e.g. unborn humans or humans left braindead after accidents. However he wrote about animals (meaning non-rational biological agents). For animals he wrote that there is no duty to treat them as ends, but that unnecessary cruelty to animals would still make us worse.
A theoretical case of a non human living being with the ability of moral choice (called "humanity" by Kant) would however imply a duty to treat them as an end (and a duty for them to treat humans as an end).
Since Kant did not write about human lives without the ability for rational thought, it is a matter of opinion and interpretation how to apply his ethics. And that's not a good topic for this site.
Typically an indirect duty exists to the rationally enabled supporters of such a being (parents, children, friends, partners), who may have a strong emotional attachment that we'd have to respect as if we were in their situation.