Is it ethical for a company that starts selling prosthetics to the military to start also selling explosive weapons?
No, a company that is built upon the foundation of helping the lives of soldiers who have lost their limbs due to explosive weapons would be considered unethical if they decide to also start selling the explosive weapons which caused said wounds in the first place.
Do you happen to know of any specific lingo used by defense contractors to describe similar behavior? Here are some examples: poisoning the patient, an Ouroboros arms dealer, and wheeling and dealing.
I am not aware of a phrase used specifically by the defense industry for this phenomenon, but this would fall under the category of conflict of interest. This term is used to describe situations in which an individual or organization has competing interests or loyalties that may compromise their ability to act impartially or in the best interests of those they serve. However, Ouroboros arms dealer is certainly more poetic.