Sounds like how Telize started. Look at the first[1] article from the author. He decided to bring the service down after being contacted by a malware research company that his service is used by ransomware.

Would you like to risk someone contacting you and possibly threatening with legal action because they would believe you might have taken part in their data being held hostage?

There was a story floating around with the curl author receiving emails like that just because he showed up in credits. I personally would pull the plug from a free service immediately if I found out it was used by malware and I would have no accountability for the person I was enabling with my software.

[1] - http://www.cambus.net/adventures-in-running-a-free-public-ap...