He turned evil because all it takes to turn evil in the Star Wars universe is to have any emotion whatsoever.
Supposedly to be on the light side you have to be a detached emotionless robot.
Stub your toe, get angry just once and it is red lightsabers and black masks for the rest of your life....
LOL!!!!
He was growing powerful, and in that universe, you have to be an emotionally detached eunuch because they are afraid anything else will lead to a power-mad dictator.
And in that universe, nearly all the “dark side” end up as sterile / abstinent as the good Jedi barring Anakin creating life once.
The irony is that this is in direct opposition to human instinct - get power as the good or bad leader to attract (female) mates who want security, so you can have top pick of the mates and lots of quality offspring. Whether you’re the drug lord or the top cop, the impulse is the same. So somehow, the Jedi’s dark side is as sexually neutral as the good side.
The interesting lesson in this is akin to the “Protector” by Larry Niven - only the really aloof and almost alien without family obligations can be trusted to have and use great power, or else the animal instincts to breed and protect family take over.