Is Bitcoin bad for the environment?
By using electricity for mining operations, it will have some effects on environment, climate change and it is bad for environment. It is fact.
However, it is true to say that Bitcoin mining has less impacts on global climate change and environment than many other industries. Because Bitcoin mining uses greener electricity (power) resources than other industries.
Is Proof of Work a security?
It is more secured than smart contracts or Proof of Stake. With Proof of Work, you can not shift ownership (in hash rate) within seconds by a hack like you can do on smart contract or Proof of Stake networks.
Bitcoin Proof of Work network is more secured than other networks:
https://howmanyconfs.com/Risk will become smaller if your transactions have more confirmations.
How many bitcoin confirmations is enough?