It's not intending to do something legal, it's intending to do something illegal: Stealing their likeness. The fact you used an otherwise legal procedure to do the illegal activity doesn't make it less illegal.
In the case of acquiring a likeness, if it's done legally you acquire someone else's likeness that happens to be shared with your target.
The likeness is shared and non-unique.
If you objective is to take someone's life, there is no other pathway to the objective but their life. With likeness that isn't the case.
Hiring someone with a voice you want isn't illegal; hiring someone with a voice you want because it is similar to a voice that someone expressly denied you permission to use is illegal.
Actually, it's so foundational to the common law legal system that there's a specialized Latin term to represent the concept: mens rea (literally 'guilty mind').