Proof of this is the millions, perhaps billions of people across all of human history who had really fucked up lives beyond what we can imagine: Victims of child rape and exploitation, abuse, torture, severe bullying, outright murder attempts, fucked up physical and mental illnesses, hellish congenital defects... and who despite their unfortunate circumstances managed to be decent people, who lived fulfilling lives, who managed to regain and uphold their pride and dignity, who became not only baseline decent, but exemplary human beings, who do not excuse disgusting behavior behind past traumas.
Yes. People have the right to be sad and complain about their shitty lives the same way people have the right to be happy about their comfortable lives...but that's not a permit to be a fucking disgusting piece of shit.