Interviewing While Spawning - Offer

I got an offer. Why do I not feel excited? It's clearly beneficial career-wise and not beneficial personally. They want me to start immediately. What do I do? I've tried to get my start date pushed back a bit, but I won't get paid for my time off. Plus, I feel like a traitor to my current position and boss, especially as they've been so good to me even though my job will be changing drastically. Would they be happy to lose the extra salary? Crap. More later.