"Weasels ripped my flesh" is active voice. The weasels are the agents (the verb-ers), AND they're the subjects. "My flesh" is the patient (the verb-ee).
"My flesh was ripped by weasels" is passive. "My flesh" is still the patient, but now it's the subject
In English, the topic is usually the subject of a sentence. Then you comment on the topic with the "predicate."
"My flesh was ripped by weasels" - the topic is your flesh. You're commenting on the horrible ordeal it went through.