Is it possible a skilled tantric practitioner could force shaktipat/avesa on an unwilling and untrained person, thus causing the untrained person so much pain that it causes their vital energies to leave the body, causing an unnatural death?
I wouldn’t call it shaktipat (transferal of energy), which is intended to be an act of love and blessing. Nor is it avesa, a term (as I understand it) which means to enter with one’s consciousness into the body of someone else. Neither would I call it true tantra, which is an ancient and authentic practice of enlightenment. What you describe is, rather, black magic: using the powers of nature, and inner powers, to do harm — in this case, murdering.
Is it possible? I believe so if the victim’s will is very weak. A person with strong will and energy can defend his or her own prana and life. It might even be more possible if for some reason the victim has an inner opening and feeling of closeness to the evildoer.
There are practices to defend oneself from such attacks. At any rate, the perpetrator will pay a high karmic price for any such dark practice.
All the best,