You can try various things...
Ek sadist kayi reasons ke chalte ladke ko torture kar sakta hai.
Jisme se main reason toh yahi h ki wo sadist h aur usse torture karne me maza aata h.
But that's too cliche...
Toh uske liye aap aisa kar sakte ho ki...
Ladka bhale hi innocent h but he carries something valuable...
Aur wo cheez uss sadist ko chahiye h...
Of course! He can't kill the innocent because only the innocent knows about that thing...
So he tortures him... Iss umeed me ki kabhi toh wo bataega...
And then aap yaha ek aur suspense create kar sakti ho ki akhir aisi kon si cheez thi jiske chalte poori kahani nikal gayi but pata na lagi...
Fir aap last update me wo cheez disclose kar sakti ho ki iske peeche poori kahani ghatit hui...
Dhyaan rahe ki jitna hi desperate wo sadist h usse paane k liye utna hi desperate innocent hona chahiye uss cheez ko na dene ke liye.
This is just a plot suggestion. You can write in any way you want. (◍•ᴗ•◍)