Controversial situation.

The following gameplay situation arose: after a victorious battle, my hero had 1 Archangel remaining. The battle took place near the opponent's castle. Having a gold reserve, he spent approximately 100,000 to buy back the same hero who was defeated in the battle. He bought an army, attacked, and on the first turn, used Implosion to destroy my Archangel. Is this within the rules? In my opinion, this is a bug in the game, and it shouldn't be possible to fight with a hero who has already been killed. What do you think?