Controversial situation.

The following gameplay situation occurred: after a victorious battle, my hero had 1 Archangel remaining. The battle took place near the opponent's castle. Having gold in reserve, he spent approximately 100,000 to repurchase the same hero who was defeated in the battle. He then bought an army, attacked, and used Implosion on the first turn 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?