User Tyler Durden answered it and wrote:
The executioners devised the most cruel ways to execute the disciples, of whom Mati Das was one, in order to try to persuade Tegh Bahadur to recant.
After searching the net for a while, I found this book that agrees with that claim:
But this claim is disputed by this Wikipedia page:
No contemporary detailed accounts of the circumstances of his arrest and execution have survived either in Persian or Sikh sources. The only accounts available are those written about a 100 years later, and these accounts are conflicting.