I can add another theory: They targeted mostly people in their 70's, 80's and 90's. These are often people, who immigrated during or shortly after WWII from Europe, They are the last ones, who can warn the younger Americans, what is really going on, they know the signs of real fascism, because they had to live through it.  And to kill the elder to have not to pay for their earned social security and healthcare, we call it (originally as sarcastic joke in Germany) "socially compatible demise". This term came into existence, when the government cheated the citizens out of their pensions by raising the retirement age and started taxing the retirement payments, which was already taxed money.