What is the fate of my friend who killed a person who was deliberately afflicting him with AIDS, knowing that he had abandoned prayer, and is he included in His (the Almighty's) saying: "And whoever kills a believer intentionally, his recompense is Hell, wherein he will abide eternally, and Allah will be angry with him and will curse him and will prepare for him a great punishment"?

It is not permissible to definitively assert that a specific individual will enter Hell based on a verse of warning or any other text. Rather, the conditions must be met and the impediments absent in their regard. Not everyone whom Allah has warned with punishment will incur it, for they may be overtaken by Allah's pardon due to an impediment or a missed condition, such as in the case of unjustly consuming the orphan's property. Occupy yourself with supplicating for your companion for pardon and forgiveness, and entrust their destiny to Allah.