If you make clear that this is fiction, it isn't really a true "deepfake" which in the primary sense of the word is a depiction that intends to mislead the viewer about whether something actually happened. It is merely special effects.
If you do so without someone's consent there are several potential theories of liability.
Individuals have a right of publicity. Generally, their likeness cannot be used to make a profit without their consent, and if it is not, they are entitled to fair compensation relative to what they would have been paid if they had agreed in a typical arms length negotiation. The exact scope of this right varies from state to state.
It is conceivable, although unlikely, that this could be construed as intentional infliction of emotional distress/outrageous conduct, if it was calculated to cause distress and emotional harm to the person faked or someone who knows the person faked. The fact that this is fiction greatly reduces this risk and it would take some quite exceptional facts (e.g. secret knowledge that the person had a deep phobia of dying in a particular way) for this claim to be legally supported.
Depending upon the nature of the film, this could be deemed an implied threat, similar to burning someone's image in effigy, which could provide a basis for the issuance of a protection order and to criminal prosecution for threatening someone's life. The context of the film and its making would have to support the theory that this was intended to be a threat to the person depicted (or someone close to them). A recent Pennsylvania Supreme Court case (of a man who was subject to a previous U.S. Supreme Court case on a similar topic) addressed the issues associated with this kind of charge.
There could be a defamation claim, related to, for example, the reason that the person is killed in the movie, if politician can credibly claim that the film, while itself fictional, is impliedly making a statement about a presently existing real world material fact about the politician that damages the politician's reputation, which is not true and is made with actual malice to destroy the politician's reputation. This might happen, for example, if the overall plot is so close to reality (or a very close allegory to reality) that calling it fiction is really inaccurate, and it is really just a smear job disguised as a fictional story.
Notable, all of these claims depend upon the intent of the party making the film, and the purpose of the scene, and the larger context. You can't determine if a "deepfake" will give rise to legal consequences in a vacuum.
Usually, fictional films contain prominent disclaimers that "any similarity to real events is purely coincidental" in an effort to rebut these theories of liability.
If a law were to be put into place down the road that outlaws this
type of portrayal, would I be grandfathered into the system and not
The particular method is new, but the legal considerations are the same. People have been making snuff films falsely depicting someone's death for more than a century, and some of them have been convincing.
A new statute cannot have ex post facto effect to acts taken prior to its enactment, but there are existing statutes and common law rules that may well provide legal consequences in appropriate circumstances as noted above.