To begin, I simply posed a hypothetical to explore the topic.
Still, whether this (or any) federal officer violates a person's Fourth Amendment rights (by way of exercising excessive force) is subject to a standard of objective reasonableness under the circumstances. E.g., Graham v. Connor, 490 U.S. 386 (1989). That applies regardless of state law (on account of the Supremacy Clause).
It only applies regardless of state law to the extent that the officer is both up to and at that point acting without malice in an objectively reasonable belief that that there actions are within their lawful federal duties (not merely the policy directives and goals of the federal superiors), because otherwise Supremacy Clause immunity does not apply, and state law controls fully.
Given ICE's very narrow jurisdiction (despite their current aggressive actions and the clear approval of their federal executive superiors for that aggression) this is a real concern about their content even before the shooting.
Graham isn't an immunity case and specifically takes subjective motivation (including malice) out of the analysis. The issue is one of reasonableness of the bodily seizure.
Was it objectively reasonable under the circumstances for the ICE officer to shoot the driver in the head? Not in my view — and apparently not in the views of the other officers on the scene — based on their actions, anyway. It seems to me that excessive force was used, violating the driver's constitutional right to be free of unreasonable bodily seizure, which resulted, tragically, in her death.
Of note, too, are today's resignations of several DOJ attorneys in the Minnesota office over the refusal to advance the investigation of the ICE officer and the push to instead investigate the victim's widow, of all people.
Still, whether this (or any) federal officer violates a person's Fourth Amendment rights (by way of exercising excessive force) is subject to a standard of objective reasonableness under the circumstances. E.g., Graham v. Connor, 490 U.S. 386 (1989). That applies regardless of state law (on account of the Supremacy Clause).