It might be advisable to take their point in good faith by considering what they may be saying instead of resorting to mentioning something about US foreign policy.

Based on the definitions I can find of murder in various dictionaries, it seems almost universally defined as an unlawful act. If you choose to go by the commonly-accepted usage of the term, it would not be a murder if the killing is found to be lawful. There are other words, such as homicide, that could be used here [1, 2]. The CPS themselves (in the UK) have a whole page about this distinction.

[1] https://www.cps.gov.uk/legal-guidance/homicide-murder-and-ma...

[2] https://murphylawoffice.org/john-murphy-law-office-blog/77-w...