Next question about startups and quitting -- how long is long enough notice? I've not gotten into specifics with any other companies yet, but is 2 weeks too little time for a startup?
Stop thinking in terms of this being a 'startup'. That term means almost nothing any more, given that companies with millions in funding that have been around for >2 years are still called 'startups' in some quarters. You work for a business, and may leave. Be prepared to leave. Give them a notice that you'll work out 2 weeks to wrap up and document stuff, train a replacement, etc. Don't give any more consideration to the long term health of this org - they've not given you any consideration. To that end, don't go out of your way to be a jerk, whether that's withholding info, trashing files, installing spyware, etc (I've seen it all happen). Just be professional and get out.
It just sounds like an extraordinarily petty and ineffective way to deal with losing a team member.
How this happened was I'd handed in my notice already - 2 weeks. But... I wanted to know if I was going to get my remaining vacation time as pay or not. I'd started before there was an 'hr handbook', but there was one in place. I wrote to HR and asked, and was told "it's in the handbook". I wrote back and asked what page it was on, because I couldn't see it in mine.
I got an email back saying something to the effect of "here's your vacation that's owed you..." and that was it. 10 minutes later the guy in the same cube as me said email to me bounced. I watched him send and it bounced back saying 'mailbox invalid'. An HR rep from our building came to my desk another 10 minutes later with a box saying "I have to escort you out of the building now". She was nice about it, and didn't seem to want to do it, but did it. So... the project I was working on was left in a broken state, client emails to me bounced, and they had no clue what was going on. I left, set up my own shop, and what do you know - that client decided to jump ship and stay with me instead of being charged twice the hourly rate for the kind of service they were getting. Basically, they considered their business too important to be cut off by phone and email from the people doing the work solely on the whim of one HR person.
Yeah, that was my rant. Again, only happened to me once, but it happened, and I've seen it happen to others.
In the UK I'd tend to consider 4 weeks "normal" both in terms of what a contract would say and what I'd expect but that may be very UK specific.
There are some situations where you might want to extend it a little if you think it's in your interest (normally that would be about not leaving on bad terms if there's a chance that it might damage your reputation to go earlier or not leaving other colleagues in the lurch) but that doesn't turn weeks into months.
As someone said, this isn't personal, this is business. Take a good honest look at how they've treated you - if you think they've gone above and beyond for you then you might reciprocate but the reality is that most businesses will act in their own interest and you should do likewise. If the owners / managers get offended by this then they're really not cut out for it.