Is it bad to wake a baby having night terrors?

Yes. True night terrors will self limit in minutes (up to 15) & the kid will never remember the event. They will be frightened if a scared looking parent wakens them for no apparent reason. Nightmares tend to occur early in the morning & the kid will awaken as soon as the sense a light touch on their skin. If it takes more than that, let them be.
Depends. You may want to talk to your pediatrician, because I need more information. Is it really night terrors or nightmares or pain or something else? How old is your baby? Night terrors usually occur 2 or 3 hours after going to sleep in young children. They are typically in a deep stage of sleep, so it is hard to get them to wake up. Also, they usually don't remember the episode the next A.M.