Personality v Sparkiness Edit
The Spark seems to be separate from every other single facet of a spark's brain or personality. We have seen insane sparks and, well, not-so-insane sparks. We've seen smart sparks and dumb sparks. Sparks can be well-intentioned or cruel. Education seems to play a part -- smarter sparks have a better chance of survival. We've also gotten hints that correct fostering, encouragement of their abilities and ambitions, can determine whether or not one successfully breaks through.
It is possible that the breakthrough of a Spark needs (or is greatly facilitated by) a strong emotional event.
For example, Agatha desperately wanted to find the Von Zinzer brothers (or her locket)... so she built a clank that finds Von Zinzer brothers (or her locket); when it back at the Clay's, the clank grabbed the brother who still carried the locket. Later, she desperately needed to build a machine for the Baron... so she built a clank that built machines. Another time, she desperately wanted to rescue Othar... so she built a clank that rescues Othar. (It's reasonable to say that the Search Engine, Dingbot Prime and the Rescue Clank are all Agatha's multi-stage breakthrough; in fact it may be that she broke through so gently because she do it all at once, but over and then in her sleep).
Furthermore, it is quite possible that Zoing, presumably the result of Gilgamesh Wulfenbach´s breakthrough event, also had a strong emotional component involved. Because of what and who he was, Gil was very lonely as a kid. It appears he may have been bullied quite a bit, too. Would be reasonable to suppose that after some particularly awful day, he built himself a loyal friend.
The destructive nature of many breakthroughs could then be explained by the fact that anger and/or hatred are very strong emotions, and that they frequently lead the one experiencing them to wish to destroy whoever is the cause of said emotion.