I frequent Santa Monica at night every night, pretty much because I live here. I wouldn't consider it scary at all, even walking by the beach at night.

The only time I've been on extra guard at night was walking around the Venice beach area around 11pm, but all the loud folks staring at us pretty much left us alone, so maybe I was just not used to it.

The promenade is about as safe as you can be, or Main Street if you want something less "touristy."