Intimidate the other guy, or the flipside, to build himself up. It would help if we knew the circumstances of this disclosure. I gather from your post that they're both at war. Maybe he wants to intimidate the other guy, maybe he wants to encourage him. "They're not so tough, this is what I've done." Maybe he wants to build up his own nerve, or his own image. (Weird, yes, but some guys would think this is cool and macho, like getting into fights with the police. Or at least imagine that others would think it's cool and macho)
I would agree most with this interpretation.
Obviously 99.999999% of military men and women are honorable, decent human beings and I have nothing but respect for them.
But we're speculating on what a potential abberant might do.
Consider that some soldiers, like Rangers and sharpshooters, are highly trained in killing people silently, or from long distances. If they are good at their job, they demonstrate it by killing more people. This soldier might take that one (irrational) step further and believe that torturing people means he's Super Soldier. He would honestly think that people would respect him more for doing this.
The military is pretty good about screening out people who are sadistic or sociopathic, but they still miss some. If you are, to use the colloquialism, "one sick puppy", you might enlist in the military to have a "respectable" outlet for the violent behavior that you want to engage in, anyway. Maybe this is someone who gets a sexual thrill from killing and torturing. He would be a mass murderer if he was a civilian, but in the military he can pretend that he's normal, and stay out of jail.
A sadist would do this because he enjoys causing other people pain, and it turns him on sexually. A sociopath might do this simply because he can, because he doesn't see other people as human beings but as cardboard cutouts put on earth for his amusement.
As far as other "whys"... well, we do live in a culture that glorifies violence in video games and movies. Inevitably, there are going to be a few people who think if it's cool on screen, it will be cooler in real life.
Maybe he was emotionally, sexually or physically abused as a young child. Most survivors of abuse grow up to be strong, decent people but there are some exceptions. Maybe this soldier is just acting out his repressed rage at being abused by a male in the past.
Or, maybe he's scared sh**less and torturing the enemy is a way of dominating them and overcoming that fear, at least for a short time. This may be rooted in his own insecurity about his abilities as a solder. Maybe this is what he is afraid the enemy soldiers will do to him, if they catch him.
Or, maybe he's still a virgin (or impotent) and this behavior is rooted in his insecurities about his masculinity. Maybe his unmarried mother was always sleeping with different guys, and he is acting out that anger (and jealousy.)
It is important that he castrates males. (Raping women would be a much more common response. Not an honorable one, just a more common one.) This would indicate rage towards a male figure of some sort. It could be a particular man (his father or an abuser), men in general ( ex. if he's homosexual and doesn't accept his sexuality) or his own sexuality.
As far as why he would tell someone about it... I can see basically two motives (besides intimidation. Which I think is unlikely.) The first would be, he is reliving his "glory days" with someone he believes will be impressed by this behavior. ("I was so tough that I...") That would more likely apply to a sadist or sociopath. Or, he is haunted by the violent acts he committed during the war, and is trying to find some solace by confessing them to one of the few people he believes will understand. (This would be more likely if he did it out of fear, or because he was abused.)