It should be noted that (unlike Docker), LXD can be safely killed and upgraded without your containers dying -- which is the main problem most people have with Docker's container liveliness model (even with Docker's --live-restore there are many issues). The main reason why LXD has a daemon is that it supports lots of management features (such as live migration and clustering) which cannot easily be done without a daemon.

You can use LXC directly if you want to avoid a long-running daemon.