Why did Venus transform from being habitable to being a hellish planet?

A possible answer is that a mini-planet gave it a glancing blow from south to north around 3 billion years ago causing it to flip. This then totally smashed its tectonic plates as to allow magma to rise up as supervolcanoes, leading to greenhouse gases that pressurize its atmosphere and make it uninhabitable.

Proof is that half of Venus is missing a land mass and has volcanoes clustered in that part.

Explains The Moon and The Pacific Ocean Too

The same thing can be applied to earth to explain the 23-degree axis tilt and Pacific ring of fire.

A mini-planet probably hit the Earth to carve out the Pacific Ocean 4.5 billion years ago and create the current tectonic plates like broken eggshells.

The Pacific ring of fire is therefore a huge crater that created volcanoes like in Venus. This pulled the Americas away from Africa to create the mid-Atlantic ridge and push India into Asia.

Unlike Venus which was hit when it was already solid, the Earth was hit when it was still hot and soft. Thus the debris from Venus fell back, while the debris from the Earth formed the moon. Moreover, the mini-planet that hit Earth likely deposited microscopic life as well.

So the main difference between Venus and the Earth was the timing of the impact.

Explains The Asteroid Belt and Planetary Tilt

If Venus and Earth were hit, then it means other planets should be hit too. A complete hit probably obliterated the planet after Mars as to create the asteroid field. Mars could have had a near-miss from such a mini-planet that disrupted its magnetic field, causing it to lose its atmosphere.

Four of the eight planets including Earth have tilts of 20+ degrees. This could imply that there was a time when mini-planets stormed across our solar system from such an angle. Only Jupiter and Mercury were spared for being too big and too small respectively.