Move Wants to unit section in order to fix:

[/lib/systemd/system/atop.service:10] Unknown lvalue 'WantsÄ in section 'Install'
Create /var/log/atop on installation in order to fix:

sh[7209]: /var/log/atop/atop_20141014 - create raw file: No such file or directory

after apt-get purge and reinstallation of package.
Also remove log files on purge as per policy, chapter 10.8:

Log files should be removed when the package is purged (but not when it
is only removed). This should be done by the postrm script when it is
called with the argument purge (see Details of removal and/or
configuration purging, Section 6.8).

https://www.debian.org/doc/debian-policy/ch-files.html