The fantasy of catching a bike thief does involve a measure of violence, if truth be told. A u-lock upside the head, perhaps. At minimum, a good aggressive shove, physical expression of the sense of violation, frustration, and anger you feel at having something something stolen that’s as a special and intimate as a bicycle.
Reality, though, well, that might be a different story. Imaginary smackdowns are tempered by real-world common sense and, perhaps, pacifism. A scumbag who would steal a bike might turn out to be a scumbag who’d stomp you into the ground, too, unless you have the brawn and smashmouth experience to stand up for yourself. Fact is, getting your bike ripped off is a crappy situation, uncontrolled, potentially dangerous, and with no good option. How would you respond?