If you wanna do a kind of insane flip like that, it has to be played up kind of like a trick. One way you could do it without having to do a major rehaul is that everyone believes he's insane after killing the lich. When he goes to take the medication, it could innitiate a cut scene of some sort of the man coming over to give you the medication, then things can go down hill again. Now this is my oppinion, but a good way of having an end to the story could be like, a flip where, jack wasn't a hero, he could have spurred on the powers of evil in an attempt to take control of it (and succeed) or again, keeping the true threat a mystery, but if you do go with the true threat a mystery, then you have to drop a hint that the lich is only a minion, preferably before the final fight. another ending could be after the lich is killed, you could do a cut scene where jack goes back and tries to dig graves for everyone, but because they died from the lich's power they come back as zombies (or skeletons) and kill him, or he could fight back and turn insane in the process because the curse is virtually never ending.