I appreciate that you want to stay friends with these people, and maybe you can, but then again maybe not. It is totally unprofessional and utterly silly for someone to get angry when one of their people quits, but nonetheless I have seen it happen. I would suggest that if they react that way, maybe they aren't such good people to have as friends after all.
I really don't understand why there's any drama here at all. As a matter of course, you should have your next opportunity lined up before you inform your employer that you're thinking about leaving.
Regardless, there is no harm in saying no. It's the norm for folks that are exploring other opportunities. The only time it may be questionable is if you've already left (or been let go by) your previous employer.
Find a new role somewhere else that you like and accept it. Go back to your current team and inform them that you've enjoyed your time working with them, but you can't pass up the new opportunity.
The only problem I think you have here is that you haven't started interviewing yet. So, of course you feel aimless and uncertain about your next step.