Kantian autonomy is determined by the faculty of reason, not by rational acts
In a strictly Kantian setting, the person is autonomous. Full stop. That is because there are two aspects in the will which are competing: the strictly rational part which makes laws (rules for the willing) and is self-determination of the will (therefore autonomy) and the part which contains all the desires and habits (in German "Willkür"). The will of finite beings is free because they have both aspects and can willingly choose between them. This choice (transcendental freedom) is taken as a given as soon as a person has the potential to have reason, regardless of whether they are acting rationally in any particular situation.
If a person decides "I want to stop smoking" and does not actually accomplish that, it means two things:
The person is autonomous since they make their will a law, ie. stopping to smoke.
The person lacks strength of will, ie. is unable to enact that rule against their own desires.
What you imply is that a person who lacks strength of will is completely unable to choose otherwise. This pessimist view of determinism (and be it only in small part) is what the Kantian conception strives to argue against in the first place.
Thus, you would under no circumstances be allowed to paternalise a person simply because you decide they lack free will and autonomy in that regard just because you think they should choose differently. Who are you to decide that? If the person seeks help themselves, they chose to do so, and that is a completely different matter.
For the same reason, the murderer is assumed to be able of rational acts and hence is autonomous even if they do obviously not act rationally when they murder someone.
Note: I tried to make it clear that I do indeed speak about Kantian concepts. The contemporary takes are as muddled as it gets, with hardly a clear-cut definition at all. They fail to fundamentally address what autonomy really is and how it relates to habitual patterns / neurological states determining behaviour. Basically, you end up with the whole free-will discussion where you either are determined regardless or, if one thinks it through, are always autonomous since otherwise, compatibalism makes no sense.