Is someone who stopped playing a computer game because it contained a cross, but his sister still plays it, considered an infidel? And is someone who plays a fighting game as a soldier in an infidel state considered an infidel?

One must avoid electronic games that contain impermissible شرعية elements, whether doctrinal (عقدية) or practical (عملية), such as symbols of disbelief (الكفر), distraction from prayer, music, or exposed 'awrah (private parts). It is not permissible to play them unless they are free from prohibitions and meet the Islamic legal guidelines. What the questioner mentioned does not go beyond sin and is not disbelief, for not everyone who commits a forbidden act falls into disbelief. The questioner should hasten to treat his obsessive thoughts and not give in to them.