Causality's a bitch

"Time travel is easy mate." Said the man sitting down opposite me in the roadside café, rain pouring down outside. "Now causality? Causality's a bitch. She don't like being messed with." He grunted, as he pulled a cigarette from an inside pocket of his wet jacket. "That butterfly you stepped on in the Jurassic? No problem. If you hadn't stepped on it, some dinosaur would've. Or it would've been eaten by something at the exact moment you stepped on it. Result? No difference. Try to kill Hitler? Your gun'll jam, bullet'll be a dud or you're just a shit shot."