Can I destroy a creature after it taps to attack?
If so, may I destroy it before or after blockers are declared?
After what taps to attack? The assassin, or an enemy creature that's attacking?
If you're asking about an enemy creature, then yes. You could tap the Assassin to destroy the attacking creature (assuming it doesn't have vigilance) at any time you have priority, which could be before blockers are declared.
If you're talking about attacking with your Assassin and then using it to destroy a creature, then no. Attack or use its ability, not both (since they both require tapping).