Never heard of a dragon tearing off the head of another..but ifthe female doesn't like the male, she will rebuff any attempts he makes to mate her. If they do fight, either can be seriously injured. Thus, any new introductions should be done in a neutral place (a wide open area works good, like a living room floor or if you have one, a small room where there is not much to run and hide under.
A kiddy wading pool works good to, minus water. Keeps the dragons from suddenly taking off and getting into or under something difficult to get them back out of (like inside a base heater...my female ran into mine and up into the wall where the pipes lead into..fortunately she came back down and I heard her scratching about and I was able to get her before she ran back in).
Mating behavior is a bit rough, as the male does grab the female by the back of the neck and sometimes will shake her to get her into position. However, rarely is there any actual damage done to the female. There are plenty of videos of breeding/mating dragons on youtube, would be a good idea to watch a few and get an idea what mating behavior includes.
Typically, the male flares out his beard, bobs his head and the female may bob in return or arm wave, or completely ignore him. If the female isn't receptive, she usually just runs away when the male tries to mount her. I haven't mated dragons so I don't know all the mating behavior...but there are alot on youtube now, so plenty of samples there.
-----
PHLdyPayne