I would never use a keylogger on anyone, period. If a person used one on me and I found out, my reaction would be beyond anger, beyond ire, beyond any type of forgiveness – that person would cease to exist in my life from that point on, “dead to me” is the perfect phrase. This is such a huge, massive breach of trust and privacy that is only available thanks to today’s technology, and I find it to be an abuse beyond cheating with another person. Not to mention your keylogger might have given me a virus or jeopardized all my online info such as banking etc.
Snooping and spying is never justified. If you are at a point where you consider snooping and keylogging to be acceptable, you have crossed serious boundaries in your relationship that may never be repaired.
The proper and logical way to proceed, if you have suspicions about your relationship, is to have a serious discussion with your partner and say your concerns plain and simple. Remember the key to heated discussions: use “I” phrases and not “you” phrases, like “I do not feel respected”, not “you are a scumbag” etc. The person’s response will be enough to help steer your decisions from that point on: if they seem honest or willing to change or help with the relationship, they are most likely sincere and it’s likely you can repair the relationship. If they ignore or dismiss your concern, or get angry and do not attempt to resolve the issue, or try and accuse you of something else, your relationship stands a high chance of being in serious peril and you may need to start thinking of what your next step will be.
cherryberry, your justification of using the keylogger to find what your kids might have run into on the computer, is pretty illogical not to mention highly unlikely. If you used the keylog info to log into any website or email, your kids would have never seen these things. If you used the keylog info to see what he was chatting, again this is something your kids would have never seen.
That said, in this case you are both in the deep. Cheating from him and spying from you, as Zaku says, really are symptoms of a bigger issue: your relationship has problems which now need to be addressed.