[Unit]
Description=rTorrent for %i
After=network.target

[Service]
Type=simple
User=%i
Group=%i
WorkingDirectory=/home/%i

Modify the next line to the absolute path for rtorrent.lock, for example

ExecStartPre=-/bin/rm -f /home/%i/.session/rtorrent.lock
ExecStart=/usr/bin/rtorrent -o system.daemon.set=true
Restart=on-failure
RestartSec=3

[Install]
WantedBy=multi-user.target