>!"Momentum". 

Sounds like they'll be waiting, but never quite finish.  

>!"Moment---um---". 

Adorn atop shoulders, they define me, break me, finish me.  

>!Starts with "m," broken by "m," finished by "m."  

I can be deadly, creative, or destructive.  

>! Deadly and destructive are obvious.  Creative I have no clear idea.  

I get speared through the middle but it's more helpful than harmful.  

>!The force Vector is drawn through the center of a mass . . .  

I can be a sign of new beginnings and a mark of rebirth.  
>!Big Bang?  All momentum hits/encounters zero (immovable) momentum = birth of universe