For some I bring the gift of life
Good advice can save lives
For others I bring death instead,
Bad advice, or advice in a bad headspace, or advice about a palliative situation can lead to death
I've been known to mend relationships,
Indeed it has!
Or to tear them apart with gossip and innuendo,
Well meaning advice is often based on hearsay, or gossip or innuendo presented as advice, and can then be wildly wrong and harmful to marriages, and other relationships......
Some think of me as a wild rockstar,
There is a band called "The Advice".
But I prefer to be a known as a simpleton,
"A word of simple advice". Advice is often described with that exact word, as "simple".
I am often given, occasionally stolen, rarely sold, and never borrowed,
Advice is indeed given (almost always), at times stolen (used by an unintended person who has obtained or overheard the advice that was given), not often/rarely sold (its then professional advice which is quite a bit different from everyday advice. Everyday advice is given but very rarely sold), and of course, is never borrowed (as it can't be returned).