First of all, his name is 김대건, so it should be spelled Kim Dae-gun or Dae-geon. There is no “T” sound in his name. But that’s how his name was spelled to the rules of what’s called the McCune-Reischauer Romanization of Korean Letters, which was developed in 1930s and not much in use these days. So, as imperfect as it is, we’ll go with it.