Are you kidding or not?
I'll explain it anyway.
One of the possible futures Shirou had was Archer. In this future, Shirou follows his ideal and ends up giving his life to become a guardian with the world so that he could fight for people. Instead, he only ends up killing more and more, which always went against his ideal. Because he was doing this even after his death, he began to regret everything he'd ever done, and became bitter, so he tried to become a servant in the Holy Grail War, so that he could go back and kill himself before he ever became Archer. Obviously they end up fighting.