Hi,
Welcome back to icliniq.com.
He is just 18, but we must rule out all possible reasons. Since he has always been argumentative and impulsive and has shown risk-taking behavior, this could be part of his behavior. But all of a sudden, he broke into houses and stole guns and other things. This should be evaluated in detail. As I mentioned, we should rule out mood disturbance and bipolar disorder. It is not uncommon at this age. In the past, he has been diagnosed with depression and anxiety, and also ADHD (Attention deficit hyperactivity disorder). But in psychiatry, diagnoses get revised from time to time.
The symptoms like all of the sudden breaking into houses, argumentativeness (if recently increased), risk-taking behavior (if increased recently), and if these are other symptoms like big ideas, claiming big feelings that I am always right, others are wrong, and I am superior to others, or other grandiose ideas, increased demand for money or spending, etc., are typical of mania and are seen in bipolar disorder. This should be ruled out. He was repenting his actions, but usually, sociopathic individuals do not regret their actions. They may repent to save themselves from punishment.
One more thing that is needed to be ruled out is recent substance use. If he has consumed any drug like a weed, marijuana, or any of its derivatives recently, that could be due to psychosis. Of nowhere, he broke into houses, which could be psychotic behavior. This possibility should also be kept in mind. ADHD is one diagnosis, but this can not explain his mood or behavior. He should be reassessed for that.
Yes, conduct disorder, he can fit. He might have been showing symptoms of conduct disorder for some years, and in conduct disorder, an individual might steal and do illegal things. Substance use further deteriorates the symptoms. But he feels terrible about what he did, which is uncommon in conduct disorder. This means he is aware of the consequences of his actions. He might be feeling bad to save himself, but you have mentioned that he was genuinely repenting.
I hope this helps.
Regards.