Whether you pay him or not is not the issue. It is whether he made an illegal copy. Since software is copyright protected as soon as it is written, that would answer the question. He or she infringed ythe owner's copyright. But almost certainly the package contained a licensing agreement, probably a shrink-wrap license, that prohibited making a copy, so he or she also has broken that license, and probably is no longer entitled to use the program.
Now, as for you, when you start the program your computer loads from hard drive to memory, and that makes a copy. Although few courts have ruled on whether that is making a copy under the copyright law, those that have have mostly said that is making a copy, so the answer is No, you cannot legally use it.
Then again, when you create a game with this program you will not want people ripping you off either.
Licensed in Maryland. Information here is general, does not create a lawyer-client relationship, nor is it a substitute for consulting with an experienced attorney on the specifics of your situation.