I planned to make some tutorial videos on NMAP, Metasploit or similar tools.

Which platform should I prefer? YouTube or udemy?

Note: I am not planning for monetize

Not udemy. I learned to hate their predatory business model. Besides, there are better ways of monetizing your content if you decide to do so some day.

I'd pick YouTube or Vimeo too.

Twitch?

@raghulm just deliver good content that wud be interesting, informative and engaging for ur audience. As viewer/subscriber I'd prefer live stream where I could interact, ask questions and provide inputs. Be candid and have fun coz u chose great topics.

Maybe YouTube and Steemit?

Youtube will get you tons of views.