I've been in this situation a couple of times (unfortunately). I'll gladly help my friends in the company by talking to them, but I won't help the company proper without being a paid consultant with reasonable fees and a contract. Once I'm not an employee, I don't have to do anything and I don't let them make me feel bad about that.

You don't owe the company anything. You used to work there and now you don't. They didn't pay you enough or treat you well enough for you to stay, and now you're gone. That's too bad for them. They screwed themselves. You're not helping out because you don't work there anymore.

Maybe they'll try to intimidate you by making you feel guilty, but just stick to your message: "I don't work for you anymore". It's as simple as that. Don't argue about old times, don't debate who said what, who did what, or anything else. You don't work there anymore and you don't owe them a thing. The more you say, the more chance you have of making the situation unfriendly.

Don't worry about the lawsuits. If they don't have enough money to pay a sysadmin, they won't have enough money to even hire a lawyer to send you the initial letter. People threaten all sorts of things, but they usually forget about it when a real attorney tells them how stupid they are and how much it would cost to actually even get the attorney to think about it.