Is it true that by killing every snake, an angel is freed?

There is no basis for the discourse that links snakes with angels. Angels are honored servants who do not disobey Allah and do what they are commanded. No one has authority over them except Allah. The Almighty said: (And they say, "The Most Gracious has taken [for Himself] a son." Exalted is He! Rather, they are honored servants. They do not precede Him in speech, and they act by His command. He knows what is before them and what is behind them, and they do not intercede except for whom He approves, and they are in awe from fear of Him.) Al-Anbiya/ 26-28. The realm of angels and jinn is part of the unseen, into which it is not permissible to delve without knowledge.