Did you deserve the job of CTO? If so, then you also deserved the paycheck. If not, then be happy with the fact that you got a cushy job that will look good on your resume and leave it at that.
Seriously, this is business. It's good to be happy about the time you worked there, but don't let it cloud your judgement. I'm not saying you should go after them. I'm just saying not to hold back because you feel you owe them something that you don't.