Both Martha Jones and Rose Tyler become soldiers after the end of their time with the Doctor, and in the case of Martha, at least, he somewhat disapproves. We find out midway through season four that Martha joined UNIT as a doctor. UNIT (at least in the modern series) is a heavily militaristic institution that helps protect Earth from Aliens. Martha and the Doctor have a dialogue about whether or not the Doctor is responsible for her fate. The Doctor disavows responsibility, but is he correct?
Analysis here is rather difficult, but I believe we can outline some observations that allow us to explore the question and will provide some framework for examining Rose's transformation in a later post. First, is that during her season, Martha is involved in significantly less fighting than either Rose or Donna. In the Sontaran Stratagem she blames her outcome based on her experiences with the Doctor. There's a sense in which she's right, and a sense in which she's wrong.
Martha does become keenly aware of how bad things can get. Her experience with the Daleks and in the three part conclusion of the season involving the Master she sees just how fragile the human situation is. In particular her battle against the Master is far more difficult than anything Rose or Donna ever faced because it dragged on for years. Martha had to give a completely defeated humanity hope.
Her experiences put fear in Martha. What if the Doctor is ever unable to help? One common theme in Dr. Who is the human will to survive at any cost. That desire is strong in Martha, and perhaps the Doctor shouldn't have traveled with her for that reason. During the David Tennant era, travelling with Martha is one of the three biggest mistakes he makes, even though she was clearly the most capable assistant of the three (we'll examine the other two at a later date). There's nothing else that experience could have produced in her. So in a sense, the Doctor is to blame, he should have known better.
At the same time, she fails to learn a critical lesson, the lesson of non-violent resistance. Seven out of nine episodes have a non-violent resolution. Even in one of the two that do, The Family of Blood, the Doctor attempted a non-violent solution (hiding). In particular, the experience that hardened her most, her defeat of the Master, should have shown her how much can be accomplished without taking arms. She did not learn that lesson. Perhaps that evolutionary urge to survive was too strong to overcome.
One wonders too, if she would have turned out differently had she not been slighted repeatedly throughout the season. The Doctor, Captain Jack, and even some of the villains compare her to Rose and find her wanting. One must wonder if the Doctor had been more supportive would she have felt so on her own when returning to life on earth. Would she have had more trust in the Doctor and turned out more like Sarah Jane Smith?
 Donna Noble also asks the same thing when she meets Martha Jones in the Sontaran Stratagem.
 We should note, though, that there really wasn't any sign of Rose Tyler becoming a soldier until the very end of season one, and there the signs are faint. Donna, though, was more strongly opposed than Rose was to the use of violence, and was more consistently compassionate than season one Rose. Though we'll never know, it's hard to imagine her becoming a soldier.
 The Doctor is visibly uncomfortable when Donna charges him with turning Martha into a soldier. He's also a little testy initially in his dialogue with Martha about her role in UNIT. You can tell that he's not totally on board with her decision to work for them.
 Granted it's impossible to see how Gridlock could have had a violent solution.