The key never leaves your computer, and is only ever used inside of the browser, in order for us to sign a "Set Options"-transaction on your behalf, so you can join the pool.
So, in short -- Yes, we would like to think that you can trust us.
If you want to be 100 per cent certain,
The website source code is published on GitHub, at https://github.com/johansten/pool.futuretense.io. There you should check out inflation-pool.js, to verify that the key is only used to sign a setOptions-operation that sets your inflation destination to the pool accountId.
Stellar-sdk.min.js is retrieved from rawgit.com, which serves as a CDN/Proxy. Rawgit gets it straight from the stellar.org repository on GitHub. A "Subresource Integrity" hash is then used to make sure the file isn't modified anywhere along the way.
The pool account is (currently) set up as a 3-of-3 multi-signature account. The signers are Dzham, Powderfan, and Sacarlson from the Stellar-public Slack.