If both players are killed at the same time, do they both die?
No, it's not correct, and no, it wasn't a draw.

As you know, if they were killed at the same time, it would be a draw. But they weren't killed at the same time. MTG is a turn-based game. It's impossible to cast two instants at the same time (at least under normal circumstances), and it's completely impossible for two instants to resolve at the same time.

Whoever had priority[1] got to cast their instant. Their opponent then got a chance to cast an instant in response. The instant cast in response, being cast last, will resolve first.

The game would have ended before the first instant had a chance to resolve.




It's not possible to determine who had priority from the info you provided. Being your turn, chances are it was you unless you just finished casting a spell.