Maybe this is just an opinion you don't want, but at least I'll try to make it reasonable. :)
I think that in the absense of anyone else to own it, you have to say that "yes" the hitman owns it. It doesn't belong to the person who paid the hitman, and it doesn't belong to me. Possession is 0.9 of the law. However, saying that someone owns something is not the same as saying that he has a right to continue ownership of it. If approached by a person with legitimate authority, it is right that it should be forfeited.
(The following may sound a little legal-esque, but I am NOT a lawyer and in any case I am not talking about what is legal but what is moral.) I think maybe the distinction here is between ownership and right-of-ownership.
To use a more simple example of the difference between what I am calling right-of-ownership and just ownership, say John paid the hitman $100 to mow his lawn next week. He gave the hitman the money, but the hitman doesn't really have right-of-ownership of the money until he actually mows the lawn, and if he fails to mow the lawn, John has the right to get the $100 back, i.e. John has right-of-ownership of the $100. However once the hitman mows the lawn, he has both ownership and right-of-ownership of the $100.
In your example, the hitman has ownership--it is in his pocket. He doesn't have right-of-ownership, because he received it in payment for an immoral act.
However, since he has ownership, the only person would would have the right to take it away would be someone who HAS right-of-ownership--i.e. in this case someone with legitimate authority, on behalf of the people in general.
National parks belong to everyone--that doesn't mean they belong to individuals in a way that would let someone decide on his own to build a house and a road in a national park. In the same way, the fact that the $100 belongs to everyone does not mean that Andrew has the tight to appropriate it for his own personal use, even if that use is to give it to the charity of his choice.
To see that Andrew does not have a right to take the $100, just think of things that some people consider immoral.
Say Jane considers organized religion to be immoral. So she steals $100 from a church and gives it to Planned Parenthood. Is that OK? The difference here is merely the determination of what is immoral and what is a good charity. But looking at this example shows the problem with allowing private individuals to determine when they can commit theft. Only legitimate authority can make the determination of when someone has forfieted the right-of-ownership. (I'm not saying that, if this legitimate authority is the government, they always get it right, but at least there is some basis for their claim to act for "the people.")
And in a more simple analysis, just because the $100 doesn't belong to the hitman, doesn't mean it belongs to Andrew.
Theft is wrong. You may not do evil that good may come. Andrew may not take the $100 unless he is commissioned to do so by legitimate authority, to whom he must turn the money over once he has it.