Hamas is not the only player. PLFP, Al Aqsa brigade. Besides, it's more of a rallying call, a show of defiance if you will. It's like when Churchill called for the 'unconditional surrender' of the Third Reich while the outcome was still very much in doubt.
Hyperbole is the norm in Arab politics and when the goal is a competition of who can be the most 'hardcore' in standing up to Israel, this is what you get. The call for the destruction of Israel is not an official part of the Hamas political platform.
here you go, a wiki page.
I direct you to the following quote :
The charter's current status within Hamas is unclear. For example, Mousa Abu Marzook, the deputy of the political bureau of Hamas, in 2007 described the charter as "an essentially revolutionary document born of the intolerable conditions under occupation" in 1988. Senior British diplomat and former British ambassador to the UN Sir Jeremy Greenstock stated in early 2009 that the Hamas charter was "drawn up by a Hamas-linked imam some [twenty] years ago and has never been adopted since Hamas was elected as the Palestinian government in 2006". Greenstock also stated that Hamas is not intent on the destruction of Israel
http://en.wikipedia.org/wiki/Hamas