Headcanon: The destructive tendencies of the Heterodynes are at least partly influenced by the Doom Bell.
We're told that one of the occasions for ringing the Bell is the birth of a new Heterodyne. I'm willing to assume that this is more or less an immediate response. Live baby? Ring bell. The Castle likes literally everything involved in this event, so I don't think it would wait very long. And the Doom Bell is not only loud, but has a direct and dire effect on the sentient psyche. Which means that one of the first things this new baby experiences is a flood of existential dread, despair, and overall doom almost literally jackhammered into (generally) his consciousness. Everything will die! YOU TOO! Everything is fleeting and inconsequential and unimportant next to the inevitable DOOM.
I'm willing to venture that this is probably not a good thing to happen to an extremely plastic infant brain. Immediately upon birth. Just gonna hazard a guess.
After that, why not burn down the world and laugh in the flaming wreckage? You always knew it would come to this.
Additionally, the three Heterodynes we know were not born within hearing range of the Doom Bell - Bill and Barry were both born and raised outside Mechanicsburg, as was Agatha - are noticeably more stable and less, well, destructively evil than their ancestors. Upbringing? Yes. I'm not discounting that. But.
...before anyone starts planning the next generation of Heterodynes, perhaps they should develop some really good soundproofing first.
Or see how Agatha feels about temporarily - very temporarily! for their own good! - timestopping new babies while the Castle plays with its favorite noisemaker, I guess.
