To be fair, Percocet isn't exactly "safe" and it's
never prescribed to "help people relax". Overdoses on non-counterfeit prescription opioids happen all the time -- Often in this exact scenario, where a (presumably) opioid-naive person takes a "normal dose" of an opioid-tolerant person like a hospice patient or an addict.
Also, taking the drugs left by someone who died of an of an overdose is best done with extreme caution, or not at all.
In assigning blame, it's also worth remembering that tablet presses are also used for compounding legal vitamins, making packaging cleaners and reagents ("dissolve one tablet of cleaner in a liter of water") and so forth.