While installing icinga2 from debmon.org on a Raspberry Pi 1 running Raspbian I was getting:

Job for icinga2.service failed. See 'systemctl status icinga2.service' and 'journalctl -xn' for details.
invoke-rc.d: initscript icinga2, action "start" failed.
Jan 26 15:16:01 pi1 prepare-dirs[3995]: Could not fetch RunAsUser variable.  Error ''. Exiting.

When running the icinga2 binary directly I got

Illegal instruction

Turns out Raspbian’s armhf is not quite armhf. Luckily I had a Raspberry Pi 2 available and with the help of rpi23-gen-image I could create a Jessie image and install icinga2.