Hidden Malware in Plugin?! Need Help.

Hello.

Me & a friend and an russian guy developed an small and easy seo plugin. But now are 3 of 10 pages infected with an code. We are not sure, but on all 3 pages was our plugin installed.

Maybe its the code from the russian guy which supported us, but we are not able to find anything.

The malware insert this script in the header.php:
https://paste.ofcode.org/A2MEuQzxRp8c7wytMtJTaj/

The Plugin can be downloaded here for check:
https://www.ads-software.com/plugins/pixo-simple-seo/

I would be very happy if anyone can help us, if the plugin contains a malware or not.

Regards