Keith, Eliezer: from what I remember of Catholic doctrine (I grew up one), breaking the seal of confession is a lesser sin than murder - as murder is a mortal sin. You go straight to hell for that one, no passing go - Especially as Jesus specifically said 'do not kill' is one of the strongest commandments - but breaking the seal, IIRC, is 'just' de-frocking and excommunication (which may or may not condemn you to hell), which are only undoable by the Pope.

However, mortal sins can be forgiven, and I recall that self-defense lessens the gravity of the offense. So given the hypothetical case of a sinner who is going to kill the priest, I think the thing to do would be to kill the sinner; but in the case of the sinner killing a bunch of other people & specifically excepting the priest (so he can't claim self-defense as in the first case), that's harder. I suppose it comes down to whether you think you can convince the Pope that you were justified in breaking the seal.