Continuity mistake: In both "Attack of the Prehistoric Pokémon" and "Charizard Chills", Charizard carries Ash on his back without any trouble. However, later on in "Charizard's Burning Ambitions", Ash is suddenly too heavy for Charizard to carry.
Continuity mistake: In the early episodes of the Advanced Generation series, May is portrayed as being slightly taller than Ash. As the series moves on, May is consistently portrayed as being slightly shorter than him.
Answer: He is a good guy but just found himself in a bad situation when he worked for Lysander. He didn't want him to harm Ash and his Pokémon when he was trying to get the special bond between Ash and his Greninja to work. He does leave Lysander and go back to being a good person.