If a person asked you for $100 so that she could buy a gun to kill someone and you gave her the money—with full knowledge of her intentions—you would be an accomplice to murder.
If instead of asking you for $100, she asked you for $50 and got the other $50 from someone else, you are just as much an accomplice to murder—providing only half the money doesn’t make you only half an accomplice any more than it is possible to kill only half a person.
What if instead of asking two people for money to buy her gun, she asks several million people; and instead of one gun, she buys the largest arsenal of lethal weapons on the planet; and instead of killing one person, she kills hundreds of thousands over the course of a few decades; and instead of a person, she is the US Government?
Each year multiple thousands of people are being killed with weapons our tax dollars have purchased. But the fact that the collection of taxes is coerced and nonoptional provides us with moral cover, along with a comforting illusion of blamelessness.