WARNING: The following contains major spoilers for Hotel Transylvania 4: Transformania, streaming now on Amazon Prime Video.
In the Hotel Transylvania series, Dracula's family has been through a lot. Luckily, as they learned more about themselves, they were able to inspire others, which is why in the third film, the monster hunter Abraham Van Helsing redeemed himself and joined them. He enjoyed seeing his daughter, Ericka, fall for the vampire, and it led to them becoming one big family. However, as Hotel Transylvania: Transformania ended, it became clear Dracula's father-in-law could still doom the family due to his lack of scientific ethics.
In this movie, Abraham made a monster ray that worked both ways. It turned Mavis' husband, Johnny, into a kaiju, while accidentally converting Dracula and his buddies into humans. It then required a crystal from South America, which took them all on a perilous journey. Luckily, they retrieved it, fixed the damaged ray and turned everyone back.
But shockingly, Mavis left the ray on Ericka's airship, with Ericka and her dad also not caring about destroying it. Instead, everyone on the blimp focused on having a grand time again, but this is quite problematic. It's easy to see Abraham toying with the ray down the line, trying to perfect it. After all, he turned his body into a mix of man and machine to become immortal, so chances are he won't destroy this device as it'll allow him to keep playing God.
But even in this state, if the weapon fell into the wrong hands, it could create the apocalypse. Humans can become monsters, and as seen when Johnny lost control, they may attempt to devour the planet. As for monsters like Dracula's crew, they could be turned into humans and wouldn't be able to defend mankind.
Simply put, Abraham cannot be trusted as there are monster kids running around such as Wendy, the werewolf, and Dennis, the dhampir. If he keeps creating with no one supervising, these dangerous weapons could harm them. At the end of the day, he's merely a mad scientist running unchecked, which is what led to his guinea pig destroying the hotel too.
Clearly, disaster follows Abraham once he has access to his lab and everyone else is distracted. The thing is, if enemies break in as well, they may find items to use to doom humanity, so Dracula really should be laying ground rules down in terms of experimentation. Ericka also needs to keep her dad in check, because unfettered experiments is how he was able to use the Kraken in the previous movie to try to break Dracula's family. Abraham will never stop tinkering, whether it's with his body or in making deadly toys, so hopefully, he learns his lesson soon, as he might have to pay a dear price someday.
See how Abraham Van Helsing is still a problem in Hotel Transylvania 4: Transformania, now streaming on Amazon Prime Video.