do you believe that you are the one who should have the power to make that decision?

If you are willing to do it you got to accept the consequences that come with it. No one has that right but decisions like that are made on a daily bases. If you were left with no other option but to save one person or a group. Would you just ignore both the individual and the group? 

but you arent saving a group, you are killing a person with the promise of trying to save lifes. you are killing a person so you can try and save lives. how do you know it will work? also why should you kill a person to try save others? because you could gain the money? why shouldnt the person who has the money try to save those lives? why not try raise the money in other ways instead of murder? 

are they paying up front?

...assuming that the government was going to do absolutely nothing with said $25 million if I rejected the offer...

It's certainly an interesting question. You'd almost automatically be saving far more lives right off the bat than you'd be sacrificing. Contact a few hospitals and find some patients that need some sort of life saving treatment but can't afford it. It wouldn't take long before you saved dozens of people.

On the other hand, it's obviously incredibly immoral to take life from someone simply because someone else could use it. That's borderlining on socialist policy right there.

In the end, no, I don't think I could. It's never my place to decide who lives or dies, and it's certainly not within my realm of authority to take life from an innocent person in the hopes of saving more people with the rewards from taking said life. Perhaps someone with more socialist leanings could do something like that, but as a believer in Libertarian policies, no.

After you kill the dude or dudette, you'll get your money in a huge suitcase delivered to you at your home by the President.

is the president liberal, conservative, or third party...?

You don't know, and that's the point. Every individual has incredible potential, and to cut short one life's potential on the basis that you might be able to accomplish that much more is a gamble. Which means you can't make an informed decisions as to whether what you are doing is justified.

There's also the flip-side. Would you let someone kill you if it meant $25 million would be invested in saving lives?

If that's what my country needs me to do, I'll do it.