While a year ago, there was very little choice for people who wanted to use a messenger that is both end-to-end encrypted and decentralized, there are now several options available: XMPP with OMEMO fits the bill as does Riot and the upcoming Briar release. I recently discovered another new arrival on the scene: Delta Chat (Android only) uses your existing email account to implement a chat function and secures messages with OpenPGP, implementing the Autocrypt standard. Obvious advantages of this approach are that it is unnecessary to create another account and that you can use the app to communicate even with people that do not have the app installed. However, one disadvantage is that the encryption does not have forward secrecy. It’s also not immediately clear to me from the UI which of my messages will be encrypted and which won’t – which might make secure use much harder (but the reason might be that I have no other contacts that use the app).
I would not recommend this yet for any sensitive information, but it is an interesting concept.