Legal Question in Personal Injury in California

If you don't mind risking being accused of and arrested for 'assault and battery', depending on the circumstances. Calling police when you see a crime is usually the proper thing to do unless you are compelled to use force to save a human life, yours or others. Animals don't count.

That depends on what physically stopping him would involve. You couldn't use a great deal of force against him, but you could, say, put yourself between him and the animal or get the animal out of harm's way. Specifically what you can or can't do will depend upon the circumstances.