The normal 'i killed my parents' or 'my parents were killed' is, as stated, a normal excuse for people to not have parents at all. I myself honestly have one character I can name off the top of my head at the moment who has no known blood relatives.
Rosalva Arce, one of my PR fancharacters.
Yeah, yeah, she's a human, she shouldn't get a voice in this, shut it. Rosy did what any kid her age (14) with reasonable mind would've done in the event of a huge monster attack and being home alone. She fled on foot. The other bits of her family were caught in the attack on San Fransisco, be it falling buildings, fires, or getting trapped, and did not make it out. Rosy managed to get out because she ran. She dumped out her bookbag, grabbed some non-refrigerated food and one bottle of water, maybe a few memoirs and a change of clothes and took to fleeing on foot. She barely got out with the backpack and the clothes on her back, but she made it out. Rosy made it out by pure luck, and that is why she's managed to be here today. Even though she has adopted parents nowadays, she doesn't live with them, or barely even has a connection to them. Along with the 'barely' status, her talk on her past. Of course, she has good reason, and I wouldn't blame her for keeping silent.
Comparing the pups stories of 'i killed them all' or Rosy's brush with death, it's obvious the latter is the choice of go for a dosage of realism in such an event (Though I wouldn't expect an attack by a godzilla-sized monster anytime soon). Rosy is the only character of mine who has a story like this (Yeva, my other PR fancharacter, lives in Florida) and I plan to keep it that way.