I've been seeing so many debates about a 51% attack as well as its costs and everything else. Now, I'd like to ask this whether it should be considered a crime that is done to potentially harm / kill the network as well as destroy BTC?
It is a crime? categorically No. But the question is what incentives those actors will get after a 51%.
Can somebody, after attacking BTC, mine all the BTC in an hour or so?
Do you mean double spend?
Can everyone become helpless to just stop their mining operations completely whenever such a situation arise?
Theoretically, yes, we can be helpless, let's say that the bad actors have 10 long and the normal and true blockchain is 9 long.
What if the attacker decides to pull out USDT against BTC and sends his coins to an exchange? Will the exchange accept the coins? Can he even make nay money by conducting such a highly expensive operation? Will somebody be even interested in making money from this or the sole intention would just be the destruction of BTC as well as the network, and to gain Monopoly and establish the rules of centralization in crypto world?
Exchanges specially with fast confirmations times are the first to be vulnerable to this kind of attacks. It could be the sole purpose is to disrupt the network and at the same time make money. However, it is not that easy, you need to have the hardware needed, labor, electricity of course. So this kind of attack is not sustainable in the long run, in my opinion.