Why is CO2 bad?

It's not. Plants need it to photosynthesize. CO is very deadly though.

It's not bad, it just help earth planet to be hotter (;

CO2 is not inherently bad. We produce CO2 when we breathe, and plants use CO2 like we use oxygen, so without any CO2 they would likely all die. However, plants and other organisms can only process a certain amount of CO2, so when we as humans have factories, cars, and all sorts of other inventions that create more CO2 than the plants can use, it stays in our atmosphere. All that CO2 in our atmosphere can lead to warmer temperatures, pollution, smog, and lots of other adverse effects. So overall, CO2 isn't bad, too much of it is.

Assuming that you mean "as a greenhouse gas".

CO2 in the atmosphere slows down or blocks infrared radiation. Daytime solar energy uptake is broad spectrum: infrared, visible light, ultraviolet. Nighttime re-radiation of energy absorbed during the day is all infrared.

The disproportionate interference with nighttime re-radiation means that the Earth retains more energy, resulting in a rise of global average temperature.