When did I say I could do that?
Depending on who the victim is (as some human beings are worth others, you contend implicitly), you would kill them (and as the only 'moral choice!), because the amount of good that $25 million could be used to bring is worth more than the value of that person's life. You have then weighed that person's life against the amount of good that $25 million could be used for.
Now you have also implied that the value of an old person's life is less than a young person's and the value of a sick person's is less than that of a healthy person's. And so I ask, "who are you to weigh the worth of somebody's life?" Because here you do that. You do not know what is the nature of somebody else's life. You have no idea of its value. Your entire argument is based on the relative value of two things (i.e. utilitarianism) arguing that we can increase overall 'utility' by killing the one and gaining the $25 million. That even this is false I shall now show.
Consider first that the $25 million already existed. We are talking about reality; the money does not simply appear after you killed this innocent person. So somebody is paying you $25 million. Now, you must show me that what this other person could have used $25 million for, other than for paying you in killing this person, has less utility than paying you in killing this person. For this is the interaction we are most concerned about, that you will be paid $25 million in exchange for killing this person. Unfortunately I can think of a number of much more Utilitarian uses for this $25 million (as can you!) and so the paying of $25 million for the death of this person cannot be justified by Utilitarian arguments. In short: it does not matter that you would have $25 million, and could use it for good, as the $25 million was already used wrongly, and you should reject the $25 million and instead work to have the other person spend this money in a more responsible manner.
"But I know I would spend the money responsibly, and I am not sure about the other person!" - this does not make up for the fact that you would surely cause the other person to spend the money terribly, in killing a person, so that you could spend the money as you see fit. Furthermore, the money would not disappear after the other person spends it on not-killing somebody; other members of society, receiving portions of the money in exchange for goods, would be able to spend money however they deem fit. The money would not simply disappear if you did not receive it.