The reason electric cars are bad for the environment is the electric cars use batteries, electricity and other hazardous waste.
Electric cars need electricity to charge the batteries up and that electricity is sometimes made using natural gas, coal etc to produce the electricity.
Although some places use wind energy, hydro power, solar etc which makes the electric car better for the environment.
However electric cars while being a bit better for the environment than gasoline or diesel vehicles are not all that great for the environment like they are advertised to be.
It also requires resources to produce the electric cars as well and the batteries are expensive to replace and the batteries in the electric car need to be recycled properly or they can harm the environment and leech chemicals into our water source.