Another theory is that whistleblowers are under a lot of stress. Pre-existing emotional issues would be exacerbated, sometimes leading to suicide.
If he did legally own the weapon, it would be difficult to prove suicide isn't the cause of death without additional evidence of another person being present.
If he didn't obtain the weapon legally, that warrants an additional investigation into how he obtained the weapon (his parents' request). While it doesn't eliminate suicide as a cause of death, it is a key detail in the case.