Is it...
A ring? (Yes, this is pretty close to "a loop", but could be interpreted differently)
It sounds like they will be waiting but they'll never quite finish.
they will be waiting -> delay, they'll never quite finish -> relay, relay that you don't finish -> "r-" (bit of a stretch)
Adorn atop shoulders they will define me, break me and finish me.
A king is adorned atop his shoulders with a cape? Possibly a crown? Don't know how to finish this one...
I can be deadly, creative or destructive.
Deadly/Destructive: Ring of Fire, Creative: ...?
I get speared through the middle but it's more helpful than harmful.
Speared through the middle by a finger, but it provides support
I am a mark of new beginnings and can be a sign of rebirth.
New beginnings -> marriage, and rebirth is the circle of life.
This could use some touching up, maybe by someone wiser than me.