I've repaired mildly bent axles before. Usually, the bend is on the freewheel side of the hub. I remove, clamp in a vise with blocks of wood to protect the threads on the axle, and then screw nuts onto the end of the axle that I'll tap with a hammer. Some hitting can straighten them out, or get mostly straight. I then flip it around so the bent side is re-installed as the non-freewheel side. And yes, I've cracked axles during rides before after bending them back too much. If they're too bent, you should replace.