I wanted to do some Super Metroid speedrun, so I bought USB SNES pad, but it's really hard to get used to it. When I try to use left or right button, down or up button also works unnecessarily. As I remember long ago, usual pad wasn't like this. So, is keyboard play can be accepted as speedrun? Or do I have to buy a new-better-pad?

If you're using an emulator it'll be fine.