hero can only use 2handed weapons after Big Bang?

Nope. I met a Korean girl in KMST who was a one-handed sword user, level 156 Hero. She used Brave Slash and it had the same animation that a two-handed sword would have. It's awkward, but it works.

it's even more awkward for axe users too.....even when they swing an axe, a sword slahes through....