Is a Muslim sinful if he offers an antivirus program in a forum used by some for unlawful things like songs, and he does not forbid them from using the program for unlawful purposes?

Enjoining against the use of this program for unlawful acts is necessary as part of forbidding evil (nahy an al-munkar) and avoiding assisting therein, and to inform those who are unaware of the prohibition of this act. If it is highly probable that people will use it for unlawful purposes, then it must be withheld from those from whom such use is feared. It can be sent to those whom you trust, advising them not to give it to anyone who is likely to use it for unlawful purposes.