Discipline Priests have pretty solid offensive ability, as healers go. On the other hand, our healing, dispels and prevention are even better, so worrying about who is getting killing blows is probably counterproductive. Still, there are times, especially in the arena, where switching from support to offense can tip the scales in a big way.
My dial-a-burst routine is a macro that triggers power infusion + hyperspeed accelerators as it casts holy fire. I follow that up with mind blast and shadow word: death. That’s a ton of damage in less than 3 seconds--not something to use over and over if you’re dueling someone from 100-0, but great for knocking someone vulnerable out of a fight.
Mana burn is not the powerhouse it once was, but it is still a good offensive tool against PVE-geared casters, other priests, hunters and moonkin. Reflective shield pays off against melee.
Also keep in mind that borrowed time will last until you cast a non-instant spell, so you could shield yourself, then follow up with shadow word pain, devouring plague, penance, and holy fire/mana burn on a target, getting massive haste for every cast.
Or you could just figure all the killing blows the people you heal get count as yours.