I was godless yet I warred with God.
Some say I killed my brother,
Inside he bled a lot. [see hint 2]
Generally I had a tough reputation.
Yet to modern ears I can sound quite sweet!
Who was I and can you explain each line of my description?
"I" was a well-known historical person, hence the history tag. So, I'm disallowing purely mythical figures, although "I" was and am certainly legendary.
The clue "... my brother; inside he bled a lot." uses a common cryptic-clue technique. Note for cryptic solvers. I did not say "... my brother: twisted inside he bled a lot", instead I said, "... my brother - inside he bled a lot." This excludes "Abel"! For people who are not used to cryptic crosswords, don't worry, all will become clear when you solve the rest of it.
The sweetness aspect will perhaps be most familiar to US speakers of English although, speaking as a Brit, I'm sure people from the UK are familiar with the term from movies and TV.