if you attack with a card, indecisively, and then decide to take it back but the other player claims to have blocked and killed it already, is the creature destroyed?
If its already made it to the combat damage step, yes. Casual play can show forgiveness, like you didn't see your opponent had a {maze of ith} out and attacking is just dumb. In a tournament though, you are s.o.l.
if you allowed them to declare blocks instead of stopping them when they try to prematurely block then you finalized your decision and that card attacked. Dont let people do things before they are allowed to, make your decision and it is not final till you ask for their response or you move on to something different. In the case of attackers all are declared at the the same time and are locked in only when you ask for blockers.
At FNM I would allow a take back as long as it didn't get past a point of no return (ie you attacked with your only flier, already dealt damage then ended your turn only then realizing you're at 4 and I have an untapped {Falkenrath Aristocrat}. ) At a higher level tournament, no such luck. I won't allow take backs because I'm there to win and want to win big. If you hand me the game, your loss. If you're a new player (less than a year) I would give a little more leniency but not allow you to rewind an entire turn.
Until you ask for response(as in you declare your attacks are final) you are good. At the beginning of declare attackers phase you declare attackers. Until you have finished the step of declaring attackers(and allow for a response) you are safe to change attackers, as spells and such may only be cast after attackers are declared. If you werent even in declare blockers step, they had no right to block in the first place so its irrelevant what they say.
ok, I thought so, we had a rematch and I beat him again 18 to -3, so it's all good