I felt rather horrible once. My Imperial doesn't like the thieves guild. If he was made jarl (and I've discussed in other posts why he should have), he'd have evicted them. But Brynelf wouldn't stop stalking him, so one day he WHACKED the nasty ol' thief with his battle-axe. And then, every other decent citizen and guard started chopping at him too.
Alas, Brynelf is essential.
So he fought back. A long struggle took place. Despite my regular application of Grand Healing, four merchants died, all of them friends of mine.

The guards were hurt. And the smelly old thief was undamaged, despite having been attacked by half of Riften. Finally I used Voice of the Emperor and stopped the party. And after that, I got four inheritance letters.
So I reloaded. I figure the game cheated by making that grotty old thief indestructible. And Tiberius STILL won't join the thieves' guild.
BRYNELF: "So, lad, ready to make some coin?"
TIBERIUS: "I've GOT coin, you oaf. I've got more coin than Maven Black-briar. Now get out of my face or I'll give you a falmer blood elixir enema!"