A program that performs a useful task while simultaneously allowing destructive acts is:

Trojan horses are software programs that masquerade or disguise as regular programs, such as games, disk utilities, and even antivirus programs. But if they are run, these programs can do malicious things to your computer. For example, a Trojan horse might appear to be a computer game, but once you double-click it, the program starts writing over certain parts of your hard drive, corrupting your data. While this is certainly something you want to avoid, it is good to know that these malicious programs are only dangerous if they are given a chance to run. Also, most antivirus programs can catch Trojan horses when scanning for viruses. Unlike viruses, however, Trojan horses dont replicate themselves. Though it is possible for a Trojan horse to be attached to a virus file that spreads to multiple computers.