If bio diesel is good, why is fossil fuel bad? They have the same original source.?

neither is good for the air, but bio diesel is renewable as it is made from plants

There's a continuous cycle of carbon through the living things on earth. The change in the amount of carbon in that system is quasistatic and as it changes the ecosystem is adapting simultaneously.

Fossil fuel is carbon that that was separated from the system at a time when the system was in a different state and burning fossil fuel introduces that carbon into the system in it's current state.

An abrupt change in the amount of carbon in the system will simultaneously produce an abrupt change in the state of the ecosystem.

That means a new profile of diversity of life is now forming and that puts strain on every living thing, including us.

By the way, bio diesel isn't really good. I mean if there were about a million times fewer cars in the world, a bio diesel approach might work. But not the way things are.

The only approach I can see that really is sustainable is electric transport powered by renewable sources like sun, wind, geothermal.

If by "the same original source" you mean the Sun this is true. Fossil fuel is solar energy that has been converted to chemical bonds and sequestered for millions of years. Biofuels are the same without the millions of years.

Some foods poisonous yet if prepared in the right way are considered a delicacy. Surprisingly millions of years can make a difference. The world has a carbon cycle. Too little carbon and life and energy cycles would be stunted. But too much can be poisonous. Life is a balancing act. Nature took millions of years to come to the modern state of balance. Man has taken about 150 years to return as much carbon to the world as existed over 800,000 years ago using fossil fuels.

Bio fuels were used before the fossil fuel age and will likely be a primary source of energy again. Biofuels use present day carbon that is part of the present day carbon cycle without adding any long sleeping carbon to the cycle. Fossil fuels are "bad" because they change the mix of the carbon cycle which we generally notice as increased CO2 in the atmosphere.

They do not have the same original source.

Bio diesel is made from "fresh" organic matter that grew on a plant or animal that took CO2 out of the air to grow. So burning biofuel of any kind is just participating in the natural carbon cycle that has been in a natural balance for millions of years.

Burning petro diesel is taking petroleum out of the ground and releasing it's carbon into the air fir the first time in millions of years. This is disrupting the carbon cycle by overloading it with CO2 in the air without creating a compensating process to remove more CO2 from the air.