Venom was never a mass murderer. At least, not the Eddie Brock incarnation.
I'm not saying Eddie is completely guilt-free of the bad things he did under the Symbiote, but again, he was bonded to an alien that fed on his rage and amplified a lot of his bad tendencies. He wasn't in his right state of mind either, Murdock wasn't entirely making stuff up in that.
And to be honest, I don't really get appealing to sensibilities like that either. Like, I understand the point, but it's not like those were real people either. There is a lot, lot of characters in comics that have gone through arcs like those, and it happens because the crux of permanent continuity is that, if you want to write a story like that, you gotta carry around the continuity of everyone else before you. And that's not exclusive to Venom either. - Even Spider-Man himself had mishaps that do not fit current or older depictions of the character, so the writer in turn decides to ignore them to their benefit.