Why was a person with such tendencies having a gun ???

we don't know where he got it from plus he's rich yeah and not like you can keep tabs on a grown adult 24/7 I'm guessing

We do keep tabs on individuals who are in this condition and specially after putting so much effort to bring him back from wherever he was stuck in the space

valid but I also read other comments that said his family doesn't really take mental health issues seriously but yeah let's see, I haven't read the novel