Why do the Cylons bother to destroy the Humans? What's their reasoning? They could have just as well populated the rest of the universe and left the puny humans alone, destroying the humans should have been noted as a waste of time. Why did they choose to get involved?
One captured version of Boomer (I think it was actually Athena) explains it to Adama. She says that the Cylons believe that the humans just did not deserve to live. The implications are it's because of their inability to live up to a moral standard.
Think of it as a cleansing or purging.
Also, from the Cylon point of view, humans are a threat to them and Cylons are long-lived. If they head for another galaxy, their fear is that at some point the humans could spread and outnumber them and find them again. If that happened, it would not be their children who were threatened, it would be them. So exterminating humanity would protect the Cylons down the line.
You learn quite a bit from the the series - how the Ones (Cavil) despised being 'mere' and inefficient flesh, and their desire to avert the plan of their creators (for humans and cylons to coexist in peace).
As @TangoOversway mentions - the cylons (at least the skinjobs) had a sense of self-righteousness and being on the "one true path".
Also, there was at least one incident during the cease-fire where the colonials invaded cylon space - this could have been seen as provocation by the cylons who then arranged the pre-emptive nuclear strike against the colonies.