Why was Sirius black going to stap peter (wormtail)?

in POA. Sirius came into the bedroom with a kinfe to kill scabbers, but Sirius can turn into a dog and is a wizard, why not raggy peter around and rip him apart like a normal dog would of or why not kill him with a wand? sure he could of borrowed ron's, harry's ,deans ect. why a knife