Yes, and it requires much more than a single nuclear warhead

A single nuclear warhead is not capable of ending life as we know it, where did you read that ?

A blockchain is only as secure as the amount of work (= processing power) that goes into it. Anyone with 51% of the processing power invested in a blockchain can attack it and essentially steal from other people. For cryptocurrencies it’s a problem that solves itself, because every person that possesses some of the cryptocurrency is incentivized to mine to keep it secure (and to earn some at the same time). The more your cryptocurrency is valuable, the more people will want to mine it and the more secure it will be.

For anything other than cryptocurrencies, you can’t incentivize a huge number of people to commit computing power to secure your blockchain. So you have to protect it some other way, for example only allowing you and some trusted people to write on it. But then it doesn’t really need to be a blockchain anymore, just a write-only database (which will perform better and occupy less space).

If it requires no work to generate a block at the end of your blockchain, any attacker can generate malicious ones.

If you never use medicine that was developed with the help of animal testing I guess you could. If you do use pretty much any kind of antibiotics though, or are unfortunately diabetic and have to use insulin, then it would be pretty hypocritical.