If you want out of there, you are outta there. What your senior co-worker did is his own business and what you do with your life is yours. The people who made you an offer actually do expect you to show up and the more you drag your butt being indecisive, the less professional and business-like you look and in fact, are.
I'll point out that you were not the one who caused your employer's company culture to go down the tubes and that you are reacting to the situation that was created for you by taking the only professional, business-like option that's available to you, which is voting with your feet. And which is a hell of a lot better for everyone including your management than going postal or being thoroughly demotivated from doing any work.
Let the management sort out the consequences - worrying about the impact of your employer's actions and decisions on himself - that was never your job and it's not in your job description and it's accurately reflected in your take-home pay, so be clear eyed about your responsibilities and your management's responsibilities and stop trying to do your management's job. Get the EFF out of there, gracefully or not. Because gracefully or not, you're outta here.
If you can keep in touch with that senior co-worker who resigned, use him as a reference. In fact, ask any senior person who knows about the quality of your work to act as your reference - It DOESN'T have to be you immediate supervisor, although that would be nice.
I've been in NYC's Silicon Alley - that's its rough and tumble, hard edged high tech sector - for 13 years and I am responding to you based on that experience.