Why do Christians say it is wrong to lie?

If government goons showed up to inject your children with poison, would you not LIE to them to deceive them in order to protect your child?

I can think of dozens of examples and scenarios in which lying would save the life of someone you love.

Also, I didn't think it was too bad for Peter to deny Jesus in Jerusalem. If he had admitted to being a disciple in that place in time, they would have arrested him and possibly crucified him as well. He lied to save his earthly life so that he could continue to spread the gospel himself.