Okay, I had to answer another puzzle so I could earn the rep to attempt this protected one, so here goes... apologies if my explanations are excessive... it's a habit I learned on EL&U...
An ellipsis. Using the Merriam-Webster definition: marks or a mark (such as … ) indicating an omission (as of words) or a pause
It sounds like they will be waiting but they'll never quite finish.
Ending a sentence abruptly with an ellipsis makes it seem like you're not finished, and it can leave people waiting... [for more]
Adorn atop shoulders they will define me, break me and finish me.
An ellipsis can occur at the beginning, middle, or end of a quotation, indicating where words have been removed from that quote. To me, "adorn atop shoulders" sounds like a phrase that is encapsulated by ellipses; That is, a phrase with an ellipsis before and after it, which defines the context, breaks up the quote, and completes the phrase as delivered.
I can be deadly, creative or destructive.
Removing words from a sentence can change its meaning entirely. It can totally remove relevant context and change the meaning of the quote (deadly), it can draw attention to certain portions of a text in a helpful way (creative), or it can harm the original intent of the text (destructive).
I get speared through the middle but it's more helpful than harmful.
An ellipsis in the middle of a sentence can make a long quote more succinct and readable, which is helpful, but it does involve spearing the sentence through the middle.
I am a mark of new beginnings and can be a sign of rebirth.
By now I'm seeing this get pretty literal. I am a mark fits quite well with an ellipsis, which is literally a kind of linguistic mark, and it can indicate a new beginning or the restarting of a quote where it was left off.