The biggest killer for indigenous people is said to be infections - bacteria and viruses. Collect some in this time - maybe a cold - and go back and presence will be enough.

It depends on how you interpret “screw over the world”, but I guess “you hate everyone” implies humans. If you go back to the first humans from Africa, before spreading out, that may have the biggest impact - significant delay or suppression in development and spread, or eradication.