Rush suffered the tragic loss of his wife and takes it out on everyone. He is also supremely pissed that with all his genius it took a "kid that plays video games with no drive in life" to crack the code he couldn't.
It was the Lucian Alliance that found the planet for the Icarus Project. Maybe there was a change in leadership that led to them wanting the planet for themselves? Or they figured out what SGC was up to and wanted to get Destiny themselves?