>Is it legal to commit a felony to catch a felon?

Happens all the time. For example:

http://www.nydailynews.com/news/crime/fbi-busted-child-porn-...

Most acts aren't crimes if police do them as part of their job. They are allowed to assault and kidnap people, take their possessions, possess drugs, break traffic laws, trespass, etc.

There are exceptions, it remains to be seen if this is one of them.