Do not use the init.d file.
You are using systemd, you have use the systemd unit file and wrapper, not the init.d script.
rm /etc/init.d/haproxy
cd contrib/systemd
cp haproxy.service.in /lib/systemd/system/
systemctl daemon-reload
systemctl enable haproxy
systemctl start haproxy
systemctl status haproxy
Note: Haproxy 1.8 and later needs to be compiled with USE_SYSTEMD=1 for the provided systemd unit file to work (needs systemd header files, that would be libsystemd-dev on Debian/Ubuntu or systemd-devel on CentOS 7+ afaik).