Can a malicious Internet Service provider launch a double spend attack? If a malicious internet service provider completely controls a user's connection, can it launch a double-spend attack against the user? How much computational effort would this take?
Everything sent over the Bitcoin network is public so you have nothing to worry if your ISP monitors your connection (only related to bitcoin transactions; your privacy is compromised in other cases). The only thing which is private are your keys, but that is not sent over the network. Now, if your ISP controls everything that moves through its servers, it can definitely change the components of the transaction. However, when you create a transaction, you sign the entire transaction with your private key. So, if any component of the transaction is altered, that signature is rendered invalid and hence the entire transaction becomes invalid. So, no even if your ISP controls your entire network, it cannot alter your transaction without invalidating it. Just ensure that you keep your keys safe (cold storage like hardware wallets or paper wallet).