Yes, you are correct.

If someone has to die -> no one has died yet, and we are considering it right now; or hypothetical If someone had to die -> in that situation (in the past); or hypothetical "If someone has to die, I think it should be me." Someone must die, and I volunteer. The way you have it written, it sounds like you have regret because someone did die and it wasn't you. That is indicated by past tense "thought" so either someone died and there is no longer a need for you to die, or you no longer think so, i.e. you changed your mind about volunteering to die. Amy gives a good answer. Using "will" unambiguously indicates future.

I don't know the technical grammatical terms, but this sentence makes me think past not future. "Had" and "thought" indicates the past. It sounds to me like someone died, and this person feels guilty that it wasn't him. It would have been better if it was him in his mind. To make this present, it would read "If some has to die, I think it should be me." "has, and think" refer to now. To make it future it would read "If someone will have to die, I think it should be me." "Will have to" refers to future, and "think" refers to now because he is thinking now about what will be.